Merge: Interpreter FFI: fix running the interpreter FFI in the interpreter

Merge: Interpreter FFI: fix running the interpreter FFI in the interpreter

Let's start with a bit of context, the interpreter supports only the light FFI, a subset of the full FFI supported by the compiler. The interpreter uses an heuristic to guess which extern methods are supported. The problem of #2018, was in this heuristic. The heuristic states that if a parameter of an extern method is a standard Nit class and not an universal or an extern class, then that method probably uses the full FFI. However, sometimes this is false, like in the interpreter where instances of Nit standard classes are stored as pointer in the C code, still without using the full FFI.

To fix #2018, this PR intro a new annotation `light_ffi` to be used on extern methods wrongly detected as full FFI. It bypasses the heuristic and force the interpreter to compile and run the method. (or it tries to) This annotation should not be used otherwise, the heuristic is pretty good at detecting when a method is supported or not.

Pull-Request: #2020
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>
Reviewed-by: Jean Privat <jean@pryen.org>