Merge: Alternative intern

Improve the handling of alternative of intern methods since an extern body can be used as a fallback.

~~~
redef class Int
   fun foo is intern `{ return foo(self) `}
end
~~~

Moreover, NativeArray intern alternatives is now fixed with nitc.

There is still an issue with the extern methods of NativeArray in the interpreter.

Pull-Request: #1898
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>