Handle cyclic definitions

[fsvieira]

There is already a test with cyclic definitions currently not active until a way to handle them is decided.

The normal way to handle cyclic definitions is not allowing cyclic definitions by ignoring them or just by throwing an error.

I would prefer to make it as a feature, and treat them as equivalent definitions.
For example x = 0 or x = 0 + 0, they are the same but not exactly equal to zebradb-core.

This is obviously a simple example, I came across cyclic definitions when I was trying to define propositional logical rules, I got cyclic definitions because a variable could be two things, but logically speaking it was correct, they work as reduction rules.