Move Asset into its own package
Collect global resources in one place 1/3

This is somewhat less unappealing than having globals/singletons
everywhere, and marginally less unappealing than using Game as the
registry. Performing dependency injection and using a factory were
both rejected out of a practical necessity to chase performance.
Collect global resources in one place 2/3
Collect global resources in one place 3/3

Asset/AssetManager refactoring. This makes each asset consumer
responsible for keeping track of asset names and asset lifecycle,
which seems simultaneously messier and better encapsulated.
Fix whitespace