faster factorcantor

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear PARI developers,

I have commited a new implementation of factorcantor following Kaltofen and
Shoup which is much faster than the previous one. Indeed, it is often faster
than factormod and use less memory.

This is not yet fully tuned, though, but this is a good start.

Cheers,
Bill.