Extend lexer for SimplicityHL formatter

[ikripaka]

Add parsing of additional tokens for formatting

• add fmt feature to separate logic for formatter parsing
• edit the logic of the lexer to include other characters as well for formatting
• add FmtTokens, which stores comments, newlines and whitespace data
• mark Token, TriviaKind, FmtToken as non-exhaustive
• modify parser to ignore comments in regular mode
• separate tests into modules
• add span to most of the structs in the ast
• add tests to test correctness of both behaviours for ordinary parsing and lossless

[LesterEvSe]

Let's flatten the nested test modules in lexer.rs. Having an outer mod tests creates unnecessary nesting since the file itself already scopes the tests.

Current:

mod tests {
    mod lexer { ... }
    mod fmt_lexer { ... }
}

Proposed:

mod lexer { ... }

mod fmt_lexer { ... }

UPD: Same idea in parse.rs

[LesterEvSe]

ACK 11c8937; tested locally with just check and just check_fuzz