Proper equality expansion

[AltGr]

Equality in the backends is at the moment handled by https://github.com/CatalaLang/catala/blob/master/compiler/lcalc/expand_op.ml , which ensures comptibility between the backends, but is a quick-and-dirty placeholder.

Improvements desired:

1. lift the equality computation functions when non-trivial (currently everything is inlined)
2. add a primitive for comparing the enumeration constructors: at the moment we rely on quadratic, nested pattern-matches