Allow any unit which can be converted "AsGame" for sprite coordinates.

The Updatable & Drawable traits are type parameterized.

They now take any tuple [of matching units] where said
unit can be safely converted to a `Game` unit.
Specify coordinate units in struct definitions.

(Type hinting is used elsewhere in function signatures & bodies.)
Let Updatable::draw() convert `#to_game()`
Merge branch 'exp/generic-updatable-trait'