[project @ 1999-02-26 12:46:45 by simonm]

Fix stable pointer bug:

   - the 'keep' flag is now a pointer to the stable name object.

   - makeStableName always returns an existing stable name object
     if one exists.

   - re-use stable name table entries iff the stable name object
     becomes unreachable (takes into account generational GC now),
     and the reference count is 0.