MethodHandles: Implement array accessors / identities & constants.

These transforms need typed access to the arguments of the handle,
which has been implemented in EmulatedStackFrame (along with tests).

These implementations favour simplicity. If required, we can gain
more performance by writing them as critical JNI functions or via
additional ugliness.

Tests added in 7ba2827e6d34023c78206972a1c3c7915be6f828.

Bug: 30550796
Test: make test-art-host
Change-Id: I8c13ca19b907e84bd31e82925191b4fcb1099904