Split up `assets.rs`

[alexdewar]

It's getting rather large at this point (it's the largest .rs file we have). Let's break it up.

I think the logical way to split it would be to have all of the stuff related to the AssetPool in a submodule. We can always re-export AssetPool from the main asset module.

[alexdewar]

Marking as "on hold" because it will conflict with what I'm doing for #1100