[Checkers] more constructors

[Telmo]

Hello again.

The main problem with my previous large commit about constructors is that 
the substitute method didn't quite do what I expected, that is, to 
substitute subtypes within a type, mutating it (maybe the method should be 
renamed?). If I want to modify the annotations on a type, I must get it 
and remove or add the annotations.

I added a tentative implementation using a new class to visit two types, 
one assignable to the other, at the same time. While it does pass all 
javari, igj and framework tests, the other tests fail, and I don't 
understand the reason. Can one of you add a test couple to the framework 
expliciting why this happens, so I can keep working on it?

-Telmo