(snake-velocity-queue, snake-update-velocity)

(snake-velocity-queue, snake-update-velocity)
(snake-final-x-velocity, snake-final-y-velocity):
New variable and functions.  Store user's keypresses
into a queue and pop from the queue each subsequent turn.
(snake-update-game, snake-move-left)
(snake-move-right, snake-move-up, snake-move-down, snake-active-p)
(snake-start-game): Use that queue.
(snake-use-glyphs-flag): Renamed from snake-use-glyphs.
(snake-use-color-flag): Likewise.
(snake-mode): Rename uses of those variables.