Java 7: Replace method invocation via reflection with use of MethodHandles

[ngmr]

MethodHandles, introduced in Java 7, offer performance and type safety benefits over invoking methods by reflection.

Yoko should take advantage of this.

[ngmr]

It looks like method handles can also be used for getting and setting field values, which would mean we can get rid of the use of Unsafe without needing VarHandles (and thus maintain Java 8 source compatibility).

[ngmr]

The elimination of Unsafe was done by PR #818, as tracked by issue #806.
This issue will track using MethodHandles for other serialization/marshalling hot-paths.