Rewrite parser macros as proc_macros

[Nadrieril]

Benefits:

• the parser code is wayyy less magical
• better error messages when a comma is missing
• it is finally possible to parse directly an Expr<E> for any E
• rustfmt accepts to format the parser code
• parser macros could be published as a useful independent crate
• rule aliasing and shortcutting can now be reimplemented
• more possibilities of future extension