Better Support for Headless Testing

[randomPoison]

Right now the only way to test Gunship is to create a full running instance of the engine, including opening a window, creating a renderer, and initializing audio. For most testing and benchmarking it's not desirable to create all of these because it slows down tests and introduces too much overhead. We need some way of running tests and benchmarking specific subsystems within the engine. At the least this means being able to do headless testing on parts of the engine that don't require rendering (like collision detection and transform updates). This would need to work well with Rust's existing benchmark framework so that we can easily start running tests and get concrete information about performance.

Ultimately we'd like to replace Rust's benchmarking framework with something more robust, probably something that allows us to get hierarchical performance data from benchmarks to better determine hot spots.

[randomPoison]

A few thoughts:

• Breaking things out into separate crates makes it easier to test them in isolation.
• We could use EngineBuilder to handle the configuration for testing, e.g. to create an engine that doesn't open a window or do audio or whatever.