7498: fix radsimp
consolidate power processing in radsimp; increase coverage
Reworked the beginning of handle() in radsimp.

Before:
In [1]: e = cos(1/(1 + sqrt(2)))
In [4]: radsimp(e/sqrt(y))
Out[4]:
  ___    ⎛    1    ⎞
╲╱ y ⋅cos⎜─────────⎟
         ⎜      ___⎟
         ⎝1 + ╲╱ 2 ⎠
────────────────────
         y

Fixed this so that handle() is still called on the argument to cos.
Added a test to radsimp, for one issue in #7498.