Api updates

[brendanzab]

• Use (u8, u8) for version number
• Allow extension lists to be any type that satisfies AsRef<[&str]>
• Separate parsing and generation stages in API

Registry::new(Api::Gl, (4, 5), Profile::Core, Fallbacks::All, ["GL_ARB_debug_output"])
    .write_bindings(GlobalGenerator, &mut file)
    .unwrap();