This change adds `STInt32` as a new `SType` under the `STInteger` umbrella, with `SType` value `12`. This is the first and only `STInteger` type that supports negative values.
* Rename ASSERT to XRPL_ASSERT
* Upgrade to Anthithesis SDK 0.4.4, and use new 0.4.4 features
* automatic cast to bool, like assert
* Add instrumentation workflow to verify build with instrumentation enabled
* 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
