power series: more efficient valuation()

[rwst]

Mike Hansen in rings/power_series_ring_element.pyx:

TODO: Rewrite valuation so it is *carried* along after any
calculation, so in almost all cases f.valuation() is instant. Also,
if you add f and g and their valuations are the same, note that we
only have to look at terms at positions = f.valuation().

Component: performance

Issue created by migration from https://trac.sagemath.org/ticket/16194