Add stubs for start-jump & stop-jump events.
Remove last_facing from player state; derive it from last known movement.
Refactor input to be more `rustic`
Stub out jump logic
Add in some impl. for jump
Give jump a sense of time
Added gravity
Added a floor
Fine tuning the jump physics.

* Does not slow down `x` axis if we are in mid-air
* Checks overflow of sprite::Millis (TODO: should I change this type?)