(Use more rustic variable names in `input event match`)
Remove `extern crate sdl2` from inner source files.

(Crate is resolved through src/main.rs)
enemies/bat.rs now has a sprite loader.
Use `sprites` map in proxy update & proxy draw calls.
Add method to determine player's center as an x-coordinate.

Player's center is defined as: player's current x coordinate + 1/2 tile width.
(Where tile width is measured in game units, e.g: distance)
Yatty uses player's center to determine which way to face!

If yatty is to the left of the player, he will face east.
If yatty is to the right of the player, he will face west.