Added int && int64_t overloads for + operator

[x] Added description of change
[x] Proper file name given
[x] Added tests and example, test must pass
[ ] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
[ ] Relevant documentation/comments is changed or added
[x] PR title follows semantic
[x] Search previous suggestions before making a new one, as yours may be a duplicate.
[x] I acknowledge that all my contributions will be made under the project's license.

Notes:

ayaankhan98 / bigINT