Rewire the leaktracer to store all refs by address, not by name

Also add better diagnostics during failure
Stop various CLONE-registries from growing indefinitely
Even saner diagnotics (view under diff -w)

Now that we no longer store signs of "ref X was here but GCed" there is no
point in "No leaks of X" pass()es either
Switch to a global symtable "classdata" visitor

This not only allows us to track anything global, not only CAG stuff
but also removes a bunch of workarounda from t/52leaks.t \o/
Now that we have the tools leak-track much more stuff when XS is there