Implement AST validation

[DaniPopes]

We must verify correctness of the parsed AST.

A few examples:

• enums must have at least 1 variant and no more than 256
• continue and break are only allowed inside of a loop
• unchecked {} blocks can only be used inside of regular blocks
• placeholder statements (_;) are only allowed in modifiers
• variable declaration statements are not allowed as the body of a loop (for, while, do while), meaning they must be inside of a block

Solc is more strict in the parser, so some of these checks can be found in the parser, while others are found in visitors, like SyntaxChecker and ContractLevelChecker.

[0xalpharush]

To implement "placeholder statements (_;) are only allowed in modifiers" and maybe also #116, it'd be nice to have something like Rowan's red-green trees to get the nearest enclosing lexical scope. For example, one can see whether a given AST node is in an assembly block, block scope {}, function, contract, or file by recursively looking up its parent.

However, it's probably simplest to create a scope tree like rustc https://github.com/rust-lang/rust/blob/8adb4b30f40e6fbd21dc1ba26c3301c7eeb6de3c/compiler/rustc_hir_analysis/src/check/region.rs#L779-L782