Create version of AST for each version of Lua

[LPGhatguy]

It should be possible to validate that the given AST is valid for a given version of Lua by casting between ASTs.

For example:

let ast = parse(source);
let ast_5_1 = ast.to_version_5_1().expect("Source was not valid Lua 5.1! (maybe it used goto?)");
let new_ast = ast_5_1.to_version_agnostic(); // What do we call this?

assert_eq!(ast, new_ast);

[LPGhatguy]

The alternative to this would be to keep every version of Lua using the same AST types, but just have validation functions to check for features you can't use in your target version.

Then we might be able to avoid the maintenance hell of trying to keep all these ASTs in sync at the expense of version-specific tools having to verify that the AST doesn't contain unknown nodes.