lib: monads in lib f-mon

[ximenesyuri]

Description

Since we are pursuing a functional approach, it would be interesting to implement monads, as Either, Maybe, Loop, etc.

Suggestion

To put them into a module mon.py in the f-core lib.

[ximenesyuri]

Actually, we voted that the monads should be in a separated lib: f-mon. Said that, I will change the name and the labels of the issue.