try return a factored expr more often from simplify
denest powers in powsimp so 4**s*2**(x-2*s) -> 2**x

Should this only be done if the base is a Number?
allow factors of coeff to enter powers
keep expr as option before powsimp
mark some solve tests as slow
Factors cleanup and non-integer exponent handling
equals tries to prove an nsimplify result
use equals on numbers in sign and abs

NOTE: things like f(0) + f(1) are considered numbers, so the is_comparable
test is used instead.
AppliedUndef.is_number -> False
exp_polar related changes

* define is_number for exp_polar

* move abs after test for exp_polar (which has been changed to a test
that arg.is_Number or pure_complex rather than testing for the _prec i
attribute).
add quick exit for simplify if expr is Atom