Make sure that operators do not generate constants

Constant propagation does not resolve things like (x[0] != x[0]) as
False, which then prevents the evaluation from detecting that the
program is not actually looking at x. By removing any operator that may
map a function of x to a constant, we make sure that a program that has
"x" in its expression actually has an x-dependent output.

NOTE: This commit does not address a corner case, of exp(large-number)
being a constant (exp(10)). Maybe some programs may learn to use exp and
then a division by a constant to fabricate a constant even though "x"
appears in their string representation