Tentative structure [WIP] for interfacing Arb

This PR can be used to facilitate discussion on how to interface Arb in CSymPy. If this doesn't require a lot of polishing, let's continue future work on this, else start-over on a different PR. 

Main changes:

+ Added method n() to `Basic` class which evaluates the instance by
  calling `eval()` function.
+ Two new files were added, `eval.h` and `eval.cpp`, which contain
  the functions for evaluating CSymPy types. These files contain main
  evaluation function `eval()` and support functions.
+ `eval()` is a if-else chain that calls the correct support function
  by checking the input instance type.
Currently n() does not check for accuracy of the returned answer and loop till we get an acceptable value.
(Comment from source file)