Additional `OR` filtering test added.
Tiny task reordering.
Code review.

Reducing code duplication by using `LogicMixin`.
Dropping unnecessary code.
Filter mixins now return its actual type, not `filter_t`.

Thus, compiler can implicit convert to the `filter_t` type if it is needed.
Naming.
Infinite `or` operator combination of filtering functions can now be used.
Finalizing infinite logic operator combinations.
One task less.
Fixed GCC 4.4. warnings.