Simfmt implementation

[ikripaka]

Current algorithm works in this way:

• After parsing, we pass input from 2 channels (ordinary tokens and extended ones) into the formatter, and it processes it
• Whenever we have missing commits (buffer from is not empty) or we have an unexpected comment in some tricky span, we'll throw an error)

Comments:

• Tests are extensive and cover a lot of logic, according to the recent refactor (we have unit, integration and CI tests). The last one uses public behaviour of the CLI. Whenever errors or mistakes are found, you can create an issue.
• Span coverage can be extended on demand to handle complex insertions (comments inside a type declaration, enum path and others) (we have comments in simplicity_fmt::doc)

Current limitations:

• No trailing commas (in some cases they're omitted, or we insert them on our demand)

Need to be fixed:

• documentation completeness (document each public function for easy-to-use documentation)
• public sharing of functions and structs (our code shares too much with the public function and struct declarations)