Implement a factory method on rules (part of immutability experiments)

[jclaveau]

Some optimizations can be done during the instanciation of rules and would change their class. For example, ['field', '=><=', [3, 3]] <=> ['field', '=', 3].

[jclaveau]

https://stackoverflow.com/questions/3368830/how-to-name-factory-like-methods

[jclaveau]

#52