HeaperAsPosition protocol

[Eric Dean Tribble <tribble>]

I just noticed that HeaperAsPosition's can return the heaper.  We
should avoid this if we can.  Supplying some server with the ID of a
Heaper ought to let the server recognize the Heaper, but not operate
any of its messages.  I just checked senders of the heaper message (in
Smalltalk), and it's only used inside HeaperAsPosition (no problem),
and HashSetEntry.  I just looked at HashSetEntry and it should be
trivial to store the Heaper rather than the HeaperAsPosition.  I think
that would also be cleaner (from an object you can get its identity,
but not vice-versa).

Note that we will be heavily using HeaperAsPosition once we start
putting caching in, and I want to guarantee that objects can't be
accessed when being used as keys in a cache.

dean