[gclist] must-finalize == reference count

[Charles Fiterman]

Suppose we are supporting an object type which I will call
must-finalize. That means the program is buggy if finalizers
on this type of object are not called. And further those
finalizers should idealy be called quite quickly.

I say this object type is identical to a reference counted
object with the following restrictions.

1) Ordinary pointers will not keep this kind of object alive.
   Ideally they should not be allowed. A pointer to a must-finalize
   object is a reference count pointer or a type error.

2) No circularity is allowed. When a pointer to a must-finalize
   object is set there is a circularity check. Or at some point
   there is a circularity check. The newest must-finalize object
   may have pointers to older must finalize objects with no
   expensive check. A non must-finalize object may have pointers
   to must-finalize objects with no expensive checks. This allows
   long chains to be kept cheaply.

Now when a pointer to a must-finalize object is deleted or changed
it decrements a reference count which may delete a must-finalize object.

At program termination pointers to must-finalize objects from stack
and static areas are deleted.

This allows execution frames to have their own cleanup sections
and to point back to other execution frames. It allows a clean
and efficient implementation of exceptions in a language where
execution frames are allocated on the heap.