Lending protocol implementation (XLS-0066)

- Add the LendingProtocol amendment
- Add Loan Broker and Loan ledger objects:
- Also add new SFields, Keylet functions, and an Invariant to verify no
  illegal field modification
- Update list of "constant" fields from spec
- Also add a general check for all object types for the type and index
  fields
- refactor: Check transaction flags in preflight0
- Adds a flagMask parameter to preflight1 so that it's impossible to
  forget to check flags.
- Also adds a short hash prefix to all Transactor log messages.
- refactor: Generalize Transactor preflight:
- Derived classes no longer need to explicitly check amendments, nor
  call into preflight1 or preflight2.
- implemeng LoanBrokerSet
- Transactions: LoanDelete, LoanManage, LoanDraw, LoanPay
- LoanBrokerSet creation mostly done. Need update.
- Also added a lookup table for pseudo account fields.
- Update changed field name.
- Modify modifiable fields in an update. Note there are only two.
- Add a node field to dirLink, defaulting sfOwnerNode, so other
  relationships can be updated.
- Create some helper classes for transaction fields
- Test that they work by converting some of the existing classes
- Finish creating helper classes for JTx fields
- Also change the pseudo account field lookup to a function that uses
  a switch
- Update tests, update pseudo-account checking
- Generalize some of the Invariant checks using macro files
  - Valid ledger entry type
  - Valid new account root and pseudo account check
- Enumerate transaction privileges for invariants
  - Allows them to be defined in transactions.macro instead of needing to
    scrutinize every existing Invariant class.
  - List is not necessarily comprehensive, but does cover every check
    where more than one transaction type is involved.
- Reserve a few values between Vault and Lending for future use
- Pseudo-account improvements
  - Define pseudo-account fields with an sfield flag
  - Pseudo-account invariant checks rules whenever a pseudo-account is
    created or modified.
- Move some helper functions.
- Check the regular key in the pseudo-transaction invariant check.
- Transactor::checkSign will always fail for a pseudo-account, so even
  if someone figures out how to get a good signature, it won't work.
- Fix account creation to check both amendments
- Add a validity range for sfDebtMaximum
- Change more "failed" messages. The goal here is to be able to search
  the log for "failed" and ONLY get test failures.
- NoModifiedUnmodifiableFields and ValidPseudoAccounts
- Move the Invariants_test class into the test namespace
- Clang wants an explicit ctor to emplace in a vector
- Refactor: Add a Transactor base function to make it easier to get the
  owner reserve increment as a fee.
- Refactor: Add an overload jtx::fee(increment) to pay an owner reserve.
- Initial implementation of LoanBrokerDelete
- Generalize the LoanBroker lifecycle test
- Refactor ApplyView::dirAdd to give access to low-level operations
  - Takes a page from #5362, which may turn out to be useful!
- Start writing Loan Broker invariants and tests
  - Specifically those mentioned for LoanBrokerDelete
- Move all detail namespaces to be under ripple
  - Avoids problems with namespace collisions / ambiguous symbol issues
    with unity builds, especially when adding or removing files.
- Add LoanBrokerCoverDeposit transaction
- Add LoanBrokerCoverWithdraw transaction
- Start writing tests for LoanBrokerCover*
- Add support for `Asset` and `MPTIssue` to some `jtx` helper classes
  and functions (`balance`, `expectLine`)
- Add support for pseudo-accounts to `jtx::Account` by allowing directly
  setting the AccountID without a matching key.
- Add Asset and MPTIssue support to more jtx objects / functions
  - Unfortunately, to work around some ambiguous symbol compilation
    errors, I had to change the implicit conversion from IOU to Asset to
    a conversion from IOU to PrettyAsset, and add a more explicit
    `asset()` function. This workaround only required changing two
    existing tests, so seems acceptable.
- Ensure that an account is not deleted with an XRP balance
  - Updates the AccountRootsDeletedClean invariant
- Finish up the Loan Broker tests
- Move inclusion of Transactor headers to transactions.macro
  - Only need to update in one place when adding a new transaction.
- Start implementing LoanSet transactor
  - Add some more values and functions to make it easier to work with
    basis point values / bips.
  - Fix several earlier mistakes.
- Generalize the check*Sign functions to support CounterParty
  - checkSign, checkSingleSign, and checkMultiSign in STTx and Transactor
- Start writing Loan tests
  - Required adding support for counterparty signature to jtx framework:
    arbitrary signature field destination, multiple signer callbacks
- Get Counterparty signing working
- Add more LoanSet unit tests, added LoanBroker LoanSequence field
  - LoanSequence will prevent loan key collisions
- Change Loan object indexing, fix several broken LoanSet unit tests
  - Loan objects will now only be indexed by LoanBrokerID and
    LoanSequence, which is a new field in LoanBroker. Also changes
    Loan.Sequence to Loan.LoanSequence to match up.
  - Several tests weren't working because of `PrettyAsset` scaling. Also,
    `PrettyAsset` calculations could overflow. Made that less likely by
    changing the type of `scale_`.
  - LoanSet will fail if an account tries to loan to itself.
- Ensure that an account is not deleted with a non-zero owner count
  - Updates the AccountRootsDeletedClean invariant
- Add unit tests to create a Loan successfully
  - Fix a few field initializations in LoanSet
- Refactor issuance validity check in VaultCreate
  - Utility function: canAddHolding
  - Call canAddHolding from any transactor that call addEmptyHolding
    (LoanBrokerSet, LoanSet)
- Start implementing LoanManage transaction
  - Also add a ValidLoan invariant
- Finish `LoanManage` functionality and tests, modulo LoanDraw/Pay
- Allow existing trust lines to loan brokers to be managed (by issuer)
- Implement LoanDelete, and fix a bunch of math errors in LoanManage
- Update to match latest spec: compute interest, LoanBroker reserves
- refactor: Define getFlagsMask in the base Transactor class
  - Returns tfUniversalMask for most transactors
  - Only transactors that use other flags need to override
- Implement LoanDraw, and made good progress on related tests
- Start implementing LoanPay transaction
- Implement LoanPay & most tests
- Also add an XRPL_ASSERT_PARTS, which splits the parts of the assert message
    so I don't have to remember the proper formatting.
Start writing LoanPay transaction tests

include/xrpl/protocol/STObject.h

@@ -242,6 +242,9 @@ public:
     getFieldPathSet(SField const& field) const;
     STVector256 const&
     getFieldV256(SField const& field) const;
+    // If not found, returns an object constructed with the given field
+    STObject
+    getFieldObject(SField const& field) const;
     STArray const&
     getFieldArray(SField const& field) const;
     STCurrency const&
@@ -515,7 +518,26 @@ protected:
 // Constraint += and -= ValueProxy operators
 // to value types that support arithmetic operations
 template <typename U>
-concept IsArithmetic = std::is_arithmetic_v<U> || std::is_same_v<U, STAmount>;
+concept IsArithmeticNumber = std::is_arithmetic_v<U> ||
+    std::is_same_v<U, Number> || std::is_same_v<U, STAmount>;
+template <
+    typename U,
+    typename Value = typename U::value_type,
+    typename Unit = typename U::unit_type>
+concept IsArithmeticValueUnit =
+    std::is_same_v<U, unit::ValueUnit<Unit, Value>> &&
+    IsArithmeticNumber<Value> && std::is_class_v<Unit>;
+template <typename U, typename Value = typename U::value_type>
+concept IsArithmeticST = !IsArithmeticValueUnit<U> && IsArithmeticNumber<Value>;
+template <typename U>
+concept IsArithmetic =
+    IsArithmeticNumber<U> || IsArithmeticST<U> || IsArithmeticValueUnit<U>;
+
+template <class T, class U>
+concept Addable = requires(T t, U u) { t = t + u; };
+template <typename T, typename U>
+concept IsArithmeticCompatible =
+    IsArithmetic<typename T::value_type> && Addable<typename T::value_type, U>;
 
 template <class T>
 class STObject::ValueProxy : public Proxy<T>
@@ -535,10 +557,12 @@ public:
     // Convenience operators for value types supporting
     // arithmetic operations
     template <IsArithmetic U>
+        requires IsArithmeticCompatible<T, U>
     ValueProxy&
     operator+=(U const& u);
 
     template <IsArithmetic U>
+        requires IsArithmeticCompatible<T, U>
     ValueProxy&
     operator-=(U const& u);
 
@@ -774,6 +798,7 @@ STObject::ValueProxy<T>::operator=(U&& u)
 
 template <typename T>
 template <IsArithmetic U>
+    requires IsArithmeticCompatible<T, U>
 STObject::ValueProxy<T>&
 STObject::ValueProxy<T>::operator+=(U const& u)
 {
@@ -783,6 +808,7 @@ STObject::ValueProxy<T>::operator+=(U const& u)
 
 template <class T>
 template <IsArithmetic U>
+    requires IsArithmeticCompatible<T, U>
 STObject::ValueProxy<T>&
 STObject::ValueProxy<T>::operator-=(U const& u)
 {