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

src/libxrpl/protocol/STAmount.cpp

@@ -1353,6 +1353,30 @@ canonicalizeRoundStrict(
     }
 }
 
+STAmount
+roundToReference(
+    STAmount const value,
+    STAmount referenceValue,
+    Number::rounding_mode rounding)
+{
+    if (value.asset().native() || !value.asset().holds<Issue>())
+        return value;
+
+    NumberRoundModeGuard mg(rounding);
+    if (referenceValue.negative() != value.negative())
+        referenceValue.negate();
+
+    if (value.exponent() > referenceValue.exponent() &&
+        (value.exponent() == referenceValue.exponent() &&
+         value.mantissa() >= referenceValue.mantissa()))
+        return value;
+    // With an IOU, the total will be truncated to the precision of the
+    // larger value: referenceValue
+    STAmount const total = referenceValue + value;
+    STAmount const result = total - referenceValue;
+    return result;
+}
+
 namespace {
 
 // We need a class that has an interface similar to NumberRoundModeGuard