Changed draw for food and obs to pygame.draw.rect()
fixed check to see if food is being drawn over an existing item, to accomodate for its new size
Made it so you cannot reverse worm into itself
_efficient_draw uses erasing irrelevant positions and drawing new ones, rather than redrawing the entire screen every tick
_lr_only uses only the left and right keys for movement

Fixed a bug in _efficient_draw and _lr_only where food wouldn't draw at all