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>