Adding scaffold for directionality in player object and renderer.

Adding :dir field to player object for current direction and associated
getter/updater. Also adding fields to PlayerRenderer record and hacking
in a placeholder for rendering the direction.
Adjusting reality to left- instead of right-handed coordinate system.

Ooos. \o/
Changing player rendering to look like pacman.
Correcting use of rand-nth on set.
Correcting use of rand-nth on set.
Drafting defrecord+ macro and converting existing objects to use it.
Replacing setInterval with requestAnimationFrame for render callback.
Revamping renderers for player and exit.

Previously copied code between the player and exit renderers is being
eliminated and each has a different appearance. The rendering logic is
being updated to center each element within the space it appears.
Merge branch 'player-arrow'