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. (Look for example the pop implementation.)
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 objects.
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
performed inside the mark_item itself).