feat: rewrite using syn

[scrabsha]

This is it. We ditch most of the static analysis code and replace it with a more elegant approach, based on subtle properties of the Rust syntax.

Structure of expandable-impl will be:

src/lib.rs
    token_tree.rs
    convert.rs
    passes/mod.rs
           pass1.rs # 1 pass per file
           pass2.rs # pass is roughly one validation step
    expand/mod.rs   # submodules TBD
    parse.rs
    error.rs

[scrabsha]

Suggested change

	/// WHen analyzing the matcher, each real metavariable creates a ghost one for every higher
	/// When analyzing the matcher, each real metavariable creates a ghost one for every higher