model: add MType::lookup_fixed

Signed-off-by: Jean Privat <jean@pryen.org>
model: improve  MType::is_subtype for formal types

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add base_formal_subtype.nit

Signed-off-by: Jean Privat <jean@pryen.org>
model: document the `assert isa MClassType`

Signed-off-by: Jean Privat <jean@pryen.org>