Change hash-map to use std::hash<T> as the hash functor template

[Dar13]

This change is needed because right now only integral types can be directly used as a key in Tervel's hash map implementation due to restrictions put on the signature of the hash() method in the current functor definition.

This change would require keys' internal type be size_t which should be trivial to change. This approach also allows users of the hash map more flexibility with regard to the types of keys used in addition to the hashing algorithm they can use.