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.