Add support for concave hitboxes

[DragonMoffon]

Arcade currently only supports concave hitboxes through Pymunk. This isn't established anywhere and causes confusing behavior. Especially since the detailed hitbox algorithm creates concave hitboxes.

While some clearer docs and possibly debug warnings would be a start, we should
ideally offer a fast collision test that supports concave hitboxes. This is non-trivial, and might not be viable in pure python.

[pushfoo]

An idea for the 3.X / future releases would be:

1. Multi-hitbox
2. Partition hitboxes to ensure all are convex