Refactor `visit(const Pow&)` to treat `Exp(x)` separately.

`Exp(x)` can be evaluated much faster with `arb_exp` than using more general
`arb_pow`. Change was made after a suggestion made by Fredrik Johansson,
author of Arb.

Link to the original comment:
https://github.com/sympy/csympy/pull/322#issuecomment-62751206