make Parser::parse_fn_decl_with_self from src/parse/parser.rs public

[Dushistov]

It would be nice to mark Parser::parse_fn_decl_with_self as pub, because of in my code: https://github.com/Dushistov/rust_siwg/blob/master/src/lib.rs to use it I have to use copy/paste technique, what is sad.