For performance reasons, the unused area of storage is always left as it is when
some elements are removed.
No time is lost to clean the released area with a Void
or a 0 value. Thus, the unused area of storage may contains references of
actually unreachable objects. The following mark_native_arrays actually replace
the default behavior (the call is automatic) in order to mark only reachable
To be used _only_ inside the definition of mark_native_arrays.
Forces the garbage collector to continue the marking process on the index-th element of
the native_array. The element at index can be Void or not Void (the Void-ness test is
performed inside the mark_item itself).