Add countability for Q, Qc, and Qp

Addded countability proofs for Q, Qc and Qp.

merged

Thanks for the merge request (and sorry for the late merge, I was on vacation).

I have shortened proofs a bit, and removed some inconsistencies (see 8f7f211d).

Do you really need countability and decidability of Q, or was that just a means to an end? I removed that, since we have practically zero results about Q. However, if you really need it, I can add it, but then we need to put decidability of Q in numbers.v.

Decidability of Q is just part of the proof. I don't need it and you can remove it.

Good, then my edited merge should be sufficient :).