Implement Expression.is_nonzero(); apply to ex.__nonzero__()

[rwst]

Despite the difficulty of proving that a symbolic expression is zero it is obvious that a positive resp. negative expression is nonzero. Also no power with nonpositive exponent can be zero. This ticket exposes the nonzero logic that is already in Pynac to the user, and applies it as a shortcut in ex.__nonzero__().

Dependencies: pynac-0.7.15

CC: @pelegm

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/24352