New factoring interface

I've been using PARI/GP for a while now and it's immensely helpful with my
number theory calculations. I also use it a lot for general factorization
as it implements algorithms other than ECM and the QS. With factorization,
though, there is an issue I run into a lot: I haven't been able to find any
way to get an "incremental" factorization.

You have to get a full factorization even if you don't use all of it, which
can be a big slowdown. For example, searching for k-almost primes --
without any builtin functions, you'll need to perform a full factorization.

If some looping construct like `foreachfactor(f = n, ...)' existed, you
could write a function `is_k_almost_prime` as `(n, k) -> my(count = 0);
foreachfactor(f = n, count++; if(count > k, return(0))); 1'. In many cases,
it would be faster than `(n, k) -> bigomega(n) == k'.

Furthermore, if this returns an incremental factorization, I think there
should be a flag (or a separate function) to enable a sorted result. I
would use it mostly for counting factors, so order is not significant...
simply run factoring algorithms until you find a factor, return it, and go
again.

I know that there is at least some foundation already present for this in
GP: set debug to a value >0 and run `factorint' on some number. The factors
should just get exposed to the user.

- Ethan