Verify message sending when debug assertions are enabled

[madsmtm]

Remove the "verify_message" feature in favour of enabling it when debug_assertions are on.

This requires us to reimplement a few things from the Objective-C runtime, but hopefully this could be a step towards fully parsing type encodings (this would allow much better error messages, ideally the error would state the required change to your Encode/RefEncode implementation).

TODO:

• [ ] EncodingBox
• [ ] ...
• [ ] Verify protocol method signatures