Factors cleanup and non-integer exponent handling

I tried removing I altogether from the factors, storing it as
-1**(1/2) but then there were transform tests that failed.

denominators of Rationals are given a negative exponent. This is
not done with Expr factors -- this is not suppose to be handling
rational expressions, really.
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
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).
modify equals and is_constant

** Inline notes describe why the minimal_polynomial is not being used
at present to prove an nsimplify result. solve, however, may be able
to prove that an expression is zero by getting a self-consistent solution
when solving for a surd of the expression. An test was added for this.

** In addition, cancel is used on the expression to try facilitate
simplification to zero (and an expression that motivated the change
has been included).

** don't use equals in sign and abs because it can be very slow

Thanks to Mario Pernici for finding good examples of when the
method used in equals is going to take a long time to solve.
The discussion that I had with him is on the PR page.
simplify handling of -1 and I in Mul.flatten
define _addsort/_mulsort to identify its usage in codebase
factor_terms can remove radical from +/- base
try return a factored expr more often from simplify

add quick exit for simplify if expr is Atom
3730: use as_content_primitive and cancel

* also change the general expand to only _mexpand when computing expr2
  in simplify