Merge: Extends operators

* Add missing combined assignments: `*=`, `/=`, `%=`, `**=`, `<<=`, `>>=`
* Add missing unary + operator (why?)
* Keep operators in the AST for better messages
* Factorize some information in the AST to simplify code in other modules

Pull-Request: #1263
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>