Faster ``minpoly`` using composed sum.

Added power series functions on "ring.PolyElement" objects.
small changes in the documentation; used ``as_int``
Change version to 0.7.5-git
used ``giant_steps`` from ``mpmath``
Merge pull request #2630 from pernici/mp5

faster minpoly using composed sum
use giant_steps in expr.py
find singularities for any expression

As the note in the function indicates, `singularities` is
not trying to identify domain points, it is just identifying
values of `sym` that would set a denominator to zero. If strict
is False (default) then removable singularities are not reported:

1/(1/x - 1) is not defined for x in (0, 1) but only 1 is returned
with the default setting for `strict`.