Bump for new `rust-nightly`: extern mod is deprecated.
Gravity based jump instead of time based jump
Remove velocity `stutter` if player repeatedly presses jump
New constants & uses motion-dependent acceleration-x
Replace slow-down factor w/ friction delta.
Add new motion type `Interacting`
Impl. `is_interacting`, simplify `Interacting` sprite loader.

* Jumping | Falling already overrides `Looking: Down` w/ JUMP_DOWN_FRAME
Tweak interaction
Merge branch 'feature/clean-movement'