Add `units::Frame` and use it in `sprite.rs`
Define constants in terms of `tiles`.
Merge branch 'feature/fix-sprites'

In this commit we use our newly defined `units` to determine which sprites
to load for the player.

Constants which are offsets of tiles are defined as `units::Tile`
Frames are defined as `units::Frame`
Pixel positioning is done using the `tile_to_pixel` conversion function.