Introduce a cache for physics collision shapes.

[dgavedissian]

With Trimesh enabled for Asteroids, spawning around 1,000 asteroids (5 asteroid fields x 200 asteroids each) take 30-40 seconds on my machine. This is because building the Trimesh hull is being done for the same mesh over and over again.

This PR adds a few things:

• A unique caching key for each mesh, which is based upon the nanosecond timestamp when each mesh object is created.
• A cache for rp::SharedShape objects, using the mesh cache key combined with the scaling factor (as scaling is "baked" into a rp::SharedShape right now).

Loading times in the above scenario has now decreased to only 2 seconds, which is a 20x improvement

[IllustrisJack]

Approved but see errors in the CI @dgavedissian