msg_send should verify the type encoding of arguments and return

[SSheldon]

At runtime, msg_send! could verify that the encoding of arguments and the return type matches those specified by the Objective-C runtime. This would help catch errors from mismatched types.

[SSheldon]

This is blocked by #11.

[SSheldon]

Verifying return type encodings was dropped in 51139b9 and #11 was closed because it wouldn't have helped. This issue should be expanded to include adding verification back.