Make MarshalGeneric use ObjectReferenceValue and remove MarshalArray2

[manodasanW]

• MarshalArray2 is actually not needed as we can just update the implementation. To avoid that API addition, removing it before our release.
• Updated generic marshalers to use ObjectReferenceValue but boxing makes it such that it doesn't get as much perf improvements as the non generic marshaler scenario.
• Remove potential race in setting and reading guid value as it is not atomic set.
• Reduce the # of funcs created by moving common ones outside of the generic class which was previously causing them to JIT each time.
• Update agile reference to also accept ObjectReferenceValue since we are updating the generic marshalers.