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

[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.

[clarfonthey]

extern crate alloc inside std/lib.rs specifically sets crate::alloc to ::alloc, so, we can't use it if we keep that.

However, I believe that this is no longer necessary with the latest MSRV, so, we could probably remove it? I'll poke around.

[clarfonthey]

So, we can't remove the extern crate alloc, but we can rename it to extern crate alloc as stdalloc, which I guess is okay.

The bottom line is that the "extern prelude" can be modified by cargo and rust flags, but when modifying it in code, it has to add the crate name into the symbol namespace at the root of the crate; this is not possible to do otherwise. rust-lang/rfcs#3875 will make this not a requirement, but, for now, it is one.