Nothing prevents abi-to-sol from generating invalid identifiers

[gnidan]

Discovered while investigating #130.

Solidity has a bunch of keywords that cannot be used as identifiers for variable names. This list of keywords of course is unlikely to (and does not) match the list of keywords from other languages, like Vyper. So if a Vyper contract defines, e.g., an event event Payment:\n type: uint256, abi-to-sol will produce event Payment(uint256 type), which is invalid.

Likely the fix for this is to cross-reference variable identifiers with a list of Solidity keywords, then to append an underscore when there's a collision.