pool feature doesn't exist but is required for pool functionality

[Kixiron]

A bunch of things critical for pooling to actually be used are gated under the pool feature (ex. HashMap::with_pool(), but the pool feature doesn't exist

[jneem]

You're totally right, we're not supporting pools right now. The pool code is there because it was there when we forked from im, and I haven't deleted it because I hope to eventually resurrect it in a different form.

In more detail: even in im, the pool feature only works in conjunction with the Rc version of the crate -- the pools aren't threadsafe, and so you can't use them with the Arc version. To keep the scope down, we've actually dropped the Rc version completely for now, so pools were a casualty of that.

Rather than re-introduce pools the way they were in im, I'd like to replace it with something like the rust stdlib's allocator API (i.e., the Vector type would become Vector<T, A> where A: Allocator, like in the std Vec type).