Move crate::raw::alloc into crate::alloc2

[clarfonthey]

Ultimately, it would be ideal to make HashTable the canonical "raw" API that HashMap and HashSet are based upon.

However, the alloc module will still be needed by everything, and because there would be a name conflict with external crate alloc, we'll name it alloc2 instead and hoist it out of the raw module.

(I could have given it a much cringier name, like ourlloc, but I decided not to in this case.)

[Amanieu]

I don't understand the issue with the name conflict. Our module is crate::alloc while the crate is ::alloc.