Add a test for exp evaluation
Avoid computations involving identity elements

Two operations, 1*x and 0 + x are not performed anymore following advice from
Ondrej.

Original comments:
https://github.com/sympy/csympy/pull/322#discussion_r20239413
https://github.com/sympy/csympy/pull/322#discussion_r20239458
Merge pull request #327 from thilinarmtb/arb_refactor

Refactor