Speed up special cases of square-free factorization.

Speed up special cases of square-free factorization.

Square-free factorization of polynomials containing a factor which is
a high power P of a symbol x did scale like O(P) in space and time.
This patch introduces a shortcut in Yun's algorithm, such that the
computation is only O(1) in space and time.

This makes it possible to compute, say sqrfree(x^P + x^(P+1)) =>
(1+x)*x^P with P=123456789.

Found this to be a bottleneck while debugging one of Vitaly Magerya's
examples.