Bring up to date against current rust

This should bring gl-rs up to date. The main change is due to rust now namespacing enums. Additionally, ast::TyPath now only takes two arguments.

Please note that this technically does not build against the current master branch of rust due to an ill-defined quote_ty macro. However, once https://github.com/rust-lang/rust/pull/19387 is fully merged things should again work as expected.