When placing a Lazy(Array|Object) into the tree, copy its [ptr, len] into the parent...

When placing a Lazy(Array|Object) into the tree, copy its [ptr, len] into the parent if available, this means that if a subtree is moved from one spot to another the subtree will only need to be rewritten if modified as well (if subtree is not modified, parent can just use a reference to the existing on-disk subtree)