Replace Issue in STIssue with Asset. STIssue with MPTIssue is only used in MPT tests.
Will be used in Vault and in transactions with STIssue fields once MPT is integrated into DEX.
* Copy Antithesis SDK version 0.4.0 to directory external/
* Add build option `voidstar` to enable instrumentation with Antithesis SDK
* Define instrumentation macros ASSERT and UNREACHABLE in terms of regular C assert
* Replace asserts with named ASSERT or UNREACHABLE
* Add UNREACHABLE to LogicError
* Document instrumentation macros in CONTRIBUTING.md
