Replace all operators overloaded as member functions with non-member overloads.

[edanor]

Except operators using scalar LHS operand, all overloaded operators are implemented as members in specialized classes.

Because of that it is necessary to both define operators in abstract interface and then re-define them in every class. This additional override is exactly the same as the interface function. Defining all operators as non-member will reduce code size and code repetition.

What needs to be considered is the overhead of such operators.

This issue relates directly to Issue #25