Daniel Gröber (dxld) authored Jul 08, 2019 and Marge Bot committed Feb 17, 2021

The callback 'return_cb' allows users to be perform additional accounting
when the traversal of a subtree is completed. This is needed for example to
determine the number or total size of closures reachable from a given
closure.

This commit also makes the lifetime increase of stackElements from commit
"rts: TraverseHeap: Increase lifetime of stackElements" optional based on
'return_cb' being set enabled or not.

Note that our definition of "subtree" here includes leaf nodes. So the
invariant is that return_cb is called for all nodes in the traversal
exactly once.