upgrade solving for abs, powers, and real/imag variables

* the power-handling section was simplified to let powsimp decide
  whether the powers can be combined rather than duplicating the
  code here
use powdenest before factoring in _tsolve
better break condition in unrad

I am not sure what the exact rpt value should be at break, but the
value now is how high it had to be so that no tests would fail.
fix detection of difference of functions