move Integral.is_zero to _eval_is_zero
make log._eval_is_zero more naive: no simplification
add literal flag to evalf
modify is_comparable
use literal flag in assumption related to numbers
7642: KroneckerDelta more judicious in evalf; = 0 for nonzero diff
make is_positive and is_negative like new assumptions
modify test which takes a long time

This only takes a long time in this branch because an attempt
at evaluating the integral is attempted now...and that integral
is zero and this takes a long time in the evalf routine. Since
we are only testing the type of arguments in this test (not
the evaluation of the integral) it seems reasonable to change the test.