Remove const from comparison_counter::operator().
Remove const from comparison_counter::operator().

At some point in time, the library required comparison and projection
functions to be immutable to some degree. The philosophy has changed and I
believe that mutable comparison functions can be handled just fine. This
is why we can remove the lying const of comparison_counter::operator().

Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort