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.
keep expr as option before powsimp
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