apart and cancel are nc-aware
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

* i order to make this work, multiplicity was made to handle Rationals.
modifications to simplify

== add quick exit for simplify if expr is Atom

== 3712: try return factored expression more often

== 3730: use as_content_primitive and cancel