new bug: nitg do not perform covariance cast because of type adaptation

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

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

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add base_gen_variance3.nit

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

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

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