Polynomial remainder sequences, gcd, and resultant

Documentation on the computation of gcd's and resultant's of
polynomials via remainder sequences, with emphasis on subresultants.
Computing resultant with subresultants

Reordered the loop in the computation of subresultants to include
one more scalar subresultant, namely, the resultant. Another loop
in the resultant function can then be omitted.

Notes:

1. The subresultants and the resultant of the pair `(f,g)` as computed
   are actually those of the pair `(g,f)` if `\deg(f) < \deg(g)`.
   This may have effect on the signs, but that is harmless in general
   when the resultant is only used for eliminating a variable.
   I have not changed this to save some time in the computation.
   Correct signs may be restored afterwards if that is necessary.

2. The resultant of `(f,0)` is zero indicating that `f` and `0` have
   common roots, namely every root of `f`. However, if `f` is a
   non-zero constant, it has no roots. In this case the resultant
   should be different from zero -- in fact one, by convention.
   I have not changed this as it would break the test in
   test_euclidtools.py.
Merge pull request #7526 from jksuom/subres

Computing resultant with subresultants