lib: add `NativeArray::length`

Engines should be able to store the allocated size.
Unlike `NativeString` that is a char*, `NativeArray` is a special Nit
opaque structure.

Signed-off-by: Jean Privat <jean@pryen.org>
nitg: implements `NativeArray::length`

Signed-off-by: Jean Privat <jean@pryen.org>
niti: implements `NativeArray::length`

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

Signed-off-by: Jean Privat <jean@pryen.org>
Merge: add NativeArray::length

Add and implement native methods `NativeArray::length` and `NativeArray::to_a`

The length could be available but was not for historial reasons.
`to_a` is trivial once `length` is defined.

A future series will add `NativeArray::new` and depreciate the ugly `calloc_array`.

Note: because of a bug in nitg, the compiler in `c_src` seems to return 0 on `length` instead of aborting.

Pull-Request: #405
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>