Miscellaneous refactors and updates (#5590)

- Added a new Invariant: `ValidPseudoAccounts` which checks that all pseudo-accounts behave consistently through creation and updates, and that no "real" accounts look like pseudo-accounts (which means they don't have a 0 sequence). 
- `to_short_string(base_uint)`. Like `to_string`, but only returns the first 8 characters. (Similar to how a git commit ID can be abbreviated.) Used as a wrapped sink to prefix most transaction-related messages. More can be added later.
- `XRPL_ASSERT_PARTS`. Convenience wrapper for `XRPL_ASSERT`, which takes the `function` and `description` as separate parameters.
- `SField::sMD_PseudoAccount`. Metadata option for `SField` definitions to indicate that the field, if set in an `AccountRoot` indicates that account is a pseudo-account. Removes the need for hard-coded field lists all over the place. Added the flag to `AMMID` and `VaultID`.
- Added functionality to `SField` ctor to detect both code and name collisions using asserts. And require all SFields to have a name
- Convenience type aliases `STLedgerEntry::const_pointer` and `STLedgerEntry::const_ref`. (`SLE` is an alias to `STLedgerEntry`.)
- Generalized `feeunit.h` (`TaggedFee`) into `unit.h` (`ValueUnit`) and added new "BIPS"-related tags for future use. Also refactored the type restrictions to use Concepts.
- Restructured `transactions.macro` to do two big things
	1. Include the `#include` directives for transactor header files directly in the macro file. Removes the need to update `applySteps.cpp` and the resulting conflicts.
	2. Added a `privileges` parameter to the `TRANSACTION` macro, which specifies some of the operations a transaction is allowed to do. These `privileges` are enforced by invariant checks. Again, removed the need to update scattered lists of transaction types in various checks.
- Unit tests:
	1.  Moved more helper functions into `TestHelpers.h` and `.cpp`. 
	2. Cleaned up the namespaces to prevent / mitigate random collisions and ambiguous symbols, particularly in unity builds.
	3. Generalized `Env::balance` to add support for `MPTIssue` and `Asset`.
	4. Added a set of helper classes to simplify `Env` transaction parameter classes: `JTxField`, `JTxFieldWrapper`, and a bunch of classes derived or aliased from it. For an example of how awesome it is, check the changes `src/test/jtx/escrow.h` for how much simpler the definitions are for `finish_time`, `cancel_time`, `condition`, and `fulfillment`. 
	5. Generalized several of the amount-related helper classes to understand `Asset`s.
     6. `env.balance` for an MPT issuer will return a negative number (or 0) for consistency with IOUs.

include/xrpl/protocol/STLedgerEntry.h

@@ -26,7 +26,9 @@
 namespace ripple {
 
 class Rules;
+namespace test {
 class Invariants_test;
+}
 
 class STLedgerEntry final : public STObject, public CountedObject<STLedgerEntry>
 {
@@ -36,6 +38,8 @@ class STLedgerEntry final : public STObject, public CountedObject<STLedgerEntry>
 public:
     using pointer = std::shared_ptr<STLedgerEntry>;
     using ref = std::shared_ptr<STLedgerEntry> const&;
+    using const_pointer = std::shared_ptr<STLedgerEntry const>;
+    using const_ref = std::shared_ptr<STLedgerEntry const> const&;
 
     /** Create an empty object with the given key and type. */
     explicit STLedgerEntry(Keylet const& k);
@@ -54,7 +58,7 @@ public:
     getText() const override;
 
     Json::Value
-    getJson(JsonOptions options) const override;
+    getJson(JsonOptions options = JsonOptions::none) const override;
 
     /** Returns the 'key' (or 'index') of this item.
         The key identifies this entry's position in
@@ -84,7 +88,8 @@ private:
     void
     setSLEType();
 
-    friend Invariants_test;  // this test wants access to the private type_
+    friend test::Invariants_test;  // this test wants access to the private
+                                   // type_
 
     STBase*
     copy(std::size_t n, void* buf) const override;