Replace units.rs with (start of) `newtype` units library.

This library takes the approach of introducing _distinct types,_ and
overloading operators for convenience.
Issues b/c the units are used in places where primitives are expected
Fix up method resolution on traits.

(Traits need to be imported to call methods on them.)
Fix up uses of library & conversions.
Fix up use of units in src/collisions.rs
Fix up units in src/players.rs
Finished fixing up units:

* Impl div operator for `Tile`, use that in Game class
* Removed unused imports
* CHAR_OFFSET sadly cannot be a `Tile` and used in a constant expression.
	* (No user defined operators in constant expressions.)
Fixes up some missing units after rebase
(Fixes more things I missed in rebase)
Use `SCALE` factor to compute Game->Pixel
(Minor style nits.)
More style nits
Finish impl. `big 4` operators for `Tile` as well
Merge pull request #3 from drbawb/exp/i-newtype-units

Use tuple structs to enforce type safety of units.