validate num. variants in enum declaration

[0xalpharush]

implements "enums must have at least 1 variant and no more than 256" from #29

[0xalpharush]

The failure is due to not failing at parsing and during AST validation instead. It's odd as #121 is passing and solc succeeds parsing that case. Will need to decide whether to allow error resilient parsing or try to maintain parity with solc. IMO keeping all of this in the semantic stage makes sense for IDE, formatting, etc

[DaniPopes]

Yeah I agree, it's unfortunate that in solc the 0 check is in the parser while the >256 check is done later, since both are allowed by the grammar. I think it's fine to ignore the failing test in here with a comment pointing to this PR

solar/tools/tester/src/solc/solidity.rs

Lines 93 to 94 in bb2719a

	| "invalid_utf8_sequence"
	) {

[DaniPopes]

Thanks!