initial early draft
case for some keycodes
added function Sdlkeycode.of_string
better message for invalid_arg "Sdlmouse.of_string"
removed unneeded indentation in function Sdlscancode.to_string
added function Sdlscancode.of_string
better invalid_arg message for Sdlkeymod.of_string function
added parameter window_flags for function Sdlrender.create_window_and_renderer