Added definations and constructors in defs.hpp and some small changes…

[Aryan810]

1 -> Made the required defination. (Suggest any changes before merging)
2 -> Wrote some comments where required.
3 -> Fixed a syntax error in utils.hpp
4 -> Added "using namespace tsfqueue::__impl;" in include/blocking_mpmc_unbounded/queue.hpp
(don't know if this is the correct way, but either we do it here or user have to do it, so i thought lets do it there)
5 -> Fixed the include of utils.hpp in all defs.hpp files. i.e from 'include "utils.hpp" ' to 'include "../utils.hpp" '

• These i just added to make it easy for us to distribute work among us in team.
• Each team member can write in group which all function they can implement. If multiple people do same function, we can select the best using pull-requests.
• Some function are in defs but not in impl, the one doing those can write them.