Detect bad WeakHashMap usages where values reference keys

[pyricau]

Not sure if this should show up in a leakcanary report or be a separate tool. Maybe in Shark CLI.

If doing this as part of the normal heap breadth first traversal, we could note for each path whether they're through a WeakHashMap and what the key was. Then as we expand that path, we check if any new reference to be enqueued is the key. If yes, it's a bad WeakHashMap. Maybe tag the key reference as a "leak"? From that point forward that path should likely be marked as not interesting for looking up bad weak hash maps.

The general idea here, as with all things leakcanary, is figuring out how we can add this without doing significantly more runtime work (e.g. doing several traversals would be bad)