Add katz-rx module

[pakoito]

There is a good chance of experimenting integration with existing apps and frameworks, and RxJava seems like a natural fit. We're looking for someone to take on the following subtasks:

• create a new module depending on katz and RxJava 2.X
• add instances of Monad, Foldable and Traversable for Observable
• introduce some tests and examples of using Observable using imperative binding notation

The estimated effort is around 500-700 lines of code. The maintainers compromise to help and support whomever is completing this ticket, as it'll help assess the difficulty of applying the concepts for newcomers.

[pakoito]

Closed by #233

Syntax in #274