rust/validation.md

Fixmes:

Lexer:

• Fix is_whitespace, add more tests
• Add more thorough tests for idents for XID_Start & XID_Continue
• Validate that float and integer literals use digits only of the appropriate base, and are in range
• Validation for unclosed char literal
• Strings are completely wrong: more tests and comparison with libsyntax.
• Comment lexing is completely wrong

Parser:

• Figure out what is the expected state of attribute grammar. Token trees or something more structured? Token trees would be unfortunate: no extend selection =/