Removed the "sdl-" prefix from all procedure names. Users may add
their own prefix when importing the module. The prefix "sdl2:" is
recommended, like so:

  (use (prefix sdl2 sdl2:))
  (sdl2:init! '(everything))
  (sdl2:create-window! "Test" 0 0 600 400)
  (sdl2:delay! 1000)
  (sdl2:quit!)

Record types have the "sdl2:" prefix built into their names (e.g.
sdl2:point, sdl2:window) to avoid type name collisions with other
modules. But, the related procedures do not have a prefix.