Merge: model: add `MType::as_notnullable` to remove the `nullable` modifier

Basically replace

~~~
if m isa MNullableType then m = m.mtype
~~~

by

~~~
m = m.as_notnullable
~~~

Pull-Request: #619
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>

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

diff --cc src/typing.nit
Simple merge