(Reorganize imports)
Remove destructor & release SDL after event loop returns.
(Replace RWLocks with RefCells.)
(Remove all uses of sync)
(New inner attr. syntax.)
Merge branch 'no-destructor'
Add half-tile constant.

Since `Game` defers scaling until it is converted to pixels we
can get away with dividing the raw tile-size in half.

What I'd like to express is something like:
	`Tile(1).to_game() / Game(2.0)`
Use new half-tile constant where appropriate.
(Modules need to be public to use trait in generic type param lists.)
Allow sprite to be constructed w/ any unit that impl. `AsGame`
Load healthbar sprite
Draw UI chrome on top of map
Rough in hud-fill
Add [temporary] "number three" to the HUD
Flash HUD while player is flashing ...
Merge branch 'exp/player-hud'