parser: Syntax is trusted to have valid data

[Kijewski]

The fields of rinja_parser::Syntax are public and Ast::from_str() trusts that the input is valid, e.g. no field contains an empty string.

If a field does contain the an empty string, then in the best case the parser panics. In the worst case it could probably deadlock, in a loop advancing the input by null bytes until you kill the process.

Should the check if the syntax is sane be moved from rinja_derive::RawSyntax into rinja_parser::Syntax?

[GuillaumeGomez]

I think it'd be a good thing to check indeed.