Test of random-shaped tree: random amount of children per node, random whether a
child is branch or not.



Make an integration test that uses multi-threading to parallelize dropping of
branches.



Make benchmarks of different drop approaches: move-child-out, ManuallyDrop, and
deep_safe_drop.  And others?



To what extent should graphs be supported?  Is it possible for a non-tree graph
(DAG or cyclic), that uses some Link type that allows such (e.g. Rc with Weak),
to be given to deep_safe_drop?  What happens?  Should/can this be guarded
against if it can't be supported?



Make some tests::drop_check::* tests that use some facility for verifying that
all nodes have been dropped and in the expected order and that
main_deep_safe_drop has only been called the expected very-limited number of
times.



Read https://rust-lang.github.io/api-guidelines/ and consider applying it.