WIP: Moving internals from Vec to Cow (Starting point for discussion)

[junderw]

Fixes #285

This will allow for const creation of BigUint etc.

I started off by just doing BigUint to give an idea of what it would take. Please use this as a starting off point for discussion.

Right now I'm not looking for optimization, but just the bare minimum.

If you have a static borrow and something in the original code calls a mut method, a copy will be made, regardless of whether the mut method would have actually changed the contents of the Vec or not.