Implement primitive to allow linking variables that have different

Implement primitive to allow linking variables that have different
names.  (Previously, linked variables were required to have the same
name.)  This requires a major redesign of the cache data structures,
and since the garbage collector knows about these structures, all of
the garbage-collector files are affected too.  The new data structures
have slightly different space requirements: a cache with no references
uses one word less than previously, while a cache with references uses
three words more than previously (independent of the number of
references).

This change requires Runtime 14.190 or later.