Add facts about when a power is zero to newask

This gives the simplest example yet of something that doesn't work with ask()
(simplest in the sense that there are no logical operators involved)

>>> from sympy.assumptions.newask import *
>>> ask(Q.zero(x), Q.zero(x**2))
>>> newask(Q.zero(x), Q.zero(x**2))
True

This one probably could be made to work with the ask() handlers system, but it
would require digging through the expressions to find the x**2, and it would
be messy.