3712

- try return a factored expr more often from simplify
	addresses issue 3712
- denest powers in powsimp so 4**s*2**(x-2*s) -> 2**x
- allow factors of coeff to enter powers so powsimp(12*2**x) -> 3*2**(x + 2)
- keep expr as option before powsimp, e.g.

```
expr = (_t*exp_polar(-I*pi))**(a - 1)
powsimp forms:
_t**(a - 1)*exp_polar(-I*pi*(a - 1))
_t**(a - 1)*exp_polar(-I*pi*(a - 1))
```
- mark some solve tests as slow
	this removes about a minute from testing time
- Factors cleanup and non-integer exponent handling
	Factors works harder to keep Rational bases sorted out better and differentiates bewteen easy-to-handle numerical exponents and more difficult symbolic exponents