Introduce M.A.D. within the schema/source instance linkage

Introduce M.A.D. within the schema/source instance linkage

When a source instance is registered with a schema instance, the code
in Schema::_register_source() adds a strong ref of the source to the
schema register, and a weak schema ref to the source itself. Install
DESTROY handlers both on Schema and ResultSource to flip this setup
any time the Schema instance goes out of scope (if we somehow
increment the refcount of $self in a DESTROY, then the garbage
collection is aborted). Tested all the way back to 5.8.1 with excellent
results.

Promote the source links in both ResultSet and Row to real ResultSource
instance refs, as there's no longer any issue with memory leaks (before
a source handle would be instantiated lazily to sidestep the source
object entirely by keeping a schema ref instead).

Add freeze/thaw hooks for proper serialization of Source-containing
structures (be it Row's or ResultSet's). In another round of cleanup
ResultSourceHandle will be reduced to purely a "no schema yet"
placeholder after schema-less a freeze/thaw cycle.