Tweaked player colliders and movement

- Changed x-movement source from "GetAxis" to "GetAxisRaw". This results
in much snappier movement.
- Increased player speed.
- Changed colliders for more fluid movement: instead of a box collider
for the whole sprite, added a circle collider around the feet and a
smaller box for the rest of the sprite. This allows movement up
ramps/stairs and stops the character from getting caught on ledges and
walls.