Remove property link's u.weakening field now that it is no longer used, and
add a new u.propset field, with plans for it to store a hashtable of
properties.

A number of pieces of infrastructure are added to make this work, but we
never actually use the new type of link yet.  If we add that soon, we might
need to fix pieces of the infrastructure.  For now, everything still works
and all tests pass.