consolidate _eval_power processing from exp into Pow

The two routines should almost be the same but they weren't. Now
exp (except for some special cases as a function) defers to Pow's
version of _eval_power. Some slight differences are now seen in
test results.
allow large exponents to join in (x**y)**z, also when z=1/2
limit polar processing to base only
edit inlines and remove unused var [skip ci]
remove incorrect test
simplify Pow._eval_power
calculate sign(s) - s so full precision is used

Since we are calculating something that should be zero, using
2 digits of precision, with a guarantee that significance is
obtained, is used.
add Examples header to docstring
enumerate cases more clearly in Pow._eval_power
modify imports and add helper func comments; delete redundant test
Merge pull request #7638 from smichr/pow2

consolidate exp._eval_power and Pow._eval_power