denest powers in powsimp so 4**s*2**(x-2*s) -> 2**x

Should this only be done if the base is a Number?
3341: allow factors of coeff to enter powers

* i order to make this work, multiplicity was made to handle Rationals.
Merge pull request #2031 from smichr/3341

3341 - allow powsimp to denest and process coefficients