handle types that aren't simple C-style identifiers

[pickx]

according to the grammar, CVL types can take the form of

• C-style identifiers: examples: uint256, address, foo
• array accesses with an optional (and possibly complex) expression as the access index. examples: uint256[9], int[]
• a mapping, of the form mapping($type => $type). examples: mapping(uint256 => mathint), address => mapping(uint => uint)

currently we assume CVL types are simple C-style identifiers, which is wrong.
to implement this we probably want a newtype over a String that validates it, or even an enum (which is probably overkill - we are not making a CVL parser!)

we should check where such non-standard types may appear, since it's possible that the grammar accepts them but the compiler later rejects them.

[pickx]

improved by #8. we still don't trim away comments or whitespace, and we are too permissive in what we accept. at the very least we should accept anything legal.