Allow casting between similar Contract and Event types

[nlordell]

Implement checked casting between different contract types. The idea here is to provide a safe alternative to IERC20::at(TokenOWL.address()) (for example) that checks the ABIs of the two contracts and makes sure they are compatible.

Similarly, it would be nice to be able to generate code to convert between, say ierc20::events::Transfer and token_owl::events::Transfer (or any other two contract events with the same signature).