Initial skeleton for considering sv ref counts
Implement seen-ref counting per RT#79789
Make the tracking_hash test independant of ref counting
tweak comment in test
Drop st->min_recurse_threshold and related logic.
Rework recursion logic
better check_new_sv logic
Implement annotations on links and target nodes of multiple references
Implement extra links representing multiple references to an SV
Fix padlist handling based on Zefram's patch in RT#83904
Only recurse into SvSTASH if SvOBJECT(sv) is true.
avoid constant folding in recent perls (which makes the refcnt 2 not 1)
Fix sv_size addr notes.
added notes to TODO
Add a comment describing the overall 'node path' scheme. Give FOLLOW_* mnemonic values.
Add more safety, tracing, and comments.
Add debug tools: np_walk_all_nodes, np_debug_node_dump, and DUMP_NPATH_NODES
Clean up DUMP_NPATH_NODES
Fix up handling of addr notes
fix str_size link