Research how to get rid of syn with full feature dependency

[LukasKalbertodt]

This adds quote a bit of compile time. I don't even need Rust AST types or anything. The only reason for syn is to parse an expression delimited by a comma. I.e. $foo:expr , in declarative macro syntax. But I don't really inspect the expression and just treat it as token stream. It's not trivial to parse that yourself, however. Take write!(foo(a, b), ".."): you can't stop after the first comma.