Isolate the random-testing logic into an iszerofunc
Isolate the random-testing logic into an iszerofunc

The _find_reasonable_pivot function takes iszerofunc as a parameter.
Passing this new iszerofunc allows for much quicker, though less
reliable, determination of whether something is zero or not.
Works best for polynomials, but polynomials are the case that is
most likely to arise when computing determinants.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy