Added constants from Number and inherited classes.

* Added basic constants.h and constants.cpp
* Removed the constants from the Integer and Complex class and
added to constants
Added special symbols `E` and `pi` to constants
Added global variables from functions.cpp to constants.cpp

Resolves #272
Added `sqrt_` in constants.cpp
Move the rest of datastructures to constants.cpp
Merge pull request #273 from sushant-hiray/constants

A separate constants file to declare all the constants.
Merged #273