[refactoring] Modularization of the strategy to define assertions

[beluamat29]

Problem

Currently all assertions rely on the class Assertion. This class has a very long protocol, grouping assertions of all kinds.

Proposed solution

Propose a strategy to modularize different classes of assertions, keeping core assertions into the Assertion class. The goal is that the users can import only the assertion modules they'd like to use (like a plugin oriented architecture).