Merge: Subtype improvements

During the work with the covariance, some bugs with `is_subtype` where identified.

The PR tries to improve the code and fixes those bugs.
It also simplifies the code in the formal types and propose a new `lookup_fixed` method on `MType`.

Pull-Request: #926
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/model/model.nit
Simple merge

diff --cc src/modelize/modelize_property.nit
Simple merge