model: intro a service to get the `Pointer` type

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg*: extern classes a polymorph in Nit, and unboxed only for extern methods

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge branch 'master' into polymorphic_extern_classes

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>