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

- also rewrite re and im in terms of each other, useful for some simplifications
Sorry.  I don't understand the comment.  Currently, `exp(3*I).is_Pow` and `exp(3).is_Pow` are false.  Perhaps this is an oversite and they should be True?  However, adding `is_Pow = True` to `exp` raises errors for a number of tests.

Yes, this can be applied for any cases with base.is_real.  I will add those cases.