#4 add constants into sdl package for keysyms

[David E. Manifold]

I transcribed these from SDL_keysym.h, enjoy.
Please apply this patch into the code as it's a feature of SDL missing from cl-sdl.
This will allow you to write (= key sdl:+k-esc+) instead of (= key (char-code #\Esc)),
for example.
Thank you.

[David E. Manifold]

symbolic names for key scancodes returned from key-down event

[David E. Manifold]

bad example, it's actually sdl:+k-escape+ as I have written the patch.