Use more intern methods and add intern factories for NativeString and NativeArray

The first commits enhance the parser to accept top-level intern methods (needed for `exit`) and intern `new` factories (analogous to extern `new` factories). Thus fix #493

The rest of the series implements two intern `new` factories, one for NativeString and one for NativeArray.
However, because the compiler in c_src cannot obviously parse the new syntax yet, the standard library is not updated.
So consider the file `test_new_native.nit` as a technology preview.