Implement type coercion rules

[Arachnid]

Right now we simply fail if you try and pass a return value of one type as an argument to another function, but we should probably understand that some types of casts (like bytes to string and bytes32 to uint256) are okay. Or we could demand explicit typecasts for types that are binary-compatible, for safety?

[BlinkyStitt]

explicit typecasts sounds safest to me. Solidity itself has gone that route.