Squeak code for Arrows!

[Brian Rice]

Well, it turns out that a simple way exists to make Arrow work with
self-references without crashing in Squeak.  I simply introduce wrapper
classes for an arrow's references, and handle the order of assignment within
the Arrow class's accessor methods, so that I can have at worst two objects
that refer to each other.  A few changes in code are all that is needed to
update the basic classes, and I will release a new prototype with a
_functioning_ ArrowWorld in a day or two!

woohoo!