Reduce overheads in InvokerBytecodeGenerator

- A small portion (~5%) of the instrumented overhead when spinning MH/LF classes in InvokeBytecodeGenerator comes from creating the exact same RuntimeVisibleAnnotationsAttribute for every method. Introducing a constant or two has a small but measurable impact.
- classDesc(MemberName.class) is called ~8000 times during a build, classDesc(MethodType.class) ~900 - special casing is profitable
- class name validation in InvokerBytecodeGenerator can be narrowed down
- various minor inefficiencies