Eval Abs(exp(x*I)) as 1 if x is real

- also rewrite re and im in terms of each other
Improve handing of Abs(b**e) when b is real
fixes Abs(x)**-3 bug
reposition Pow and exp handling in Abs; add coverage test
Merge pull request #9 from smichr/abs

reposition Pow and exp handling in Abs; add coverage test
Merge pull request #8359 from zanzibar7/eval_Abs_exp

Eval Abs(exp(x*I)) as 1 if x is real
Add _eval_simplify helper for AlgebraicNumber

fixes #4401