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.

src/test/unit_test/multi_runner.cpp

@@ -30,7 +30,6 @@
 #include <vector>
 
 namespace ripple {
-namespace test {
 
 namespace detail {
 
@@ -388,6 +387,8 @@ multi_runner_base<IsParent>::add_failures(std::size_t failures)
 
 }  // namespace detail
 
+namespace test {
+
 //------------------------------------------------------------------------------
 
 multi_runner_parent::multi_runner_parent() : os_(std::cout)
@@ -645,10 +646,11 @@ multi_runner_child::on_log(std::string const& msg)
     message_queue_send(MessageType::log, s.str());
 }
 
+}  // namespace test
+
 namespace detail {
 template class multi_runner_base<true>;
 template class multi_runner_base<false>;
 }  // namespace detail
 
-}  // namespace test
 }  // namespace ripple