Add benchmarks.

[seanfisk]

[benmoran56]

If you could give me a rough idea of what all the benchmark should be stressing, I could throw something together. Ex:

Spawn X Entities, with X components each. Perform X number of queries for multiple components. etc.

Ecs seems to be fairly complete functionality wise besides the multiple component queries, so I'd be interested in helping that code be merged.

[pmrv]

I'd say benchmarks are mostly interesting for the EntityManager.{pairs_for_type,component_for_entity} functions. You probably have to toy around a bit with the number of entities and components, though.