Make op method extensible - TODO

See #348

As mentioned, the filter namespace and logic namespace were not not refactored in that PR. The reason is that, the filter namespace is still under refactoring (#336), and the logic namespace could crash if such refactoring is applied.

The logic namespace has conflicts between some `PrimitiveLogic` ops and some generic logic ops (which are typed on `T`, not `T extends Type`). For example:

```
ops().logic().greaterThan(10, 11); // should it returns BoolType or boolean?
```

To solve this problem, more work might need to be done. Since I am not currently working on this project, I file this issue just as a reminder.