last iteration through this iteration of the iteration stuff

[Eric Dean Tribble <tribble>]

MarkM and I worked out a bunch of details on the phone for the
iteration style that went by a few messages ago.  Just as a check I
went and reworked the awkward places in the Compositor and current
Iterator stuff, and it was MUCH simpler.  All the algorithmic behavior
is inline rather than off in a predicate/Fn definition.  

We decided to have heap allocated iteration objects, but they have to
be treated like Vars.  don't hand them around, hand around copies.

I'm going to try changing the hardest, nastiest iteration problem in
the Ent (the current implementation is gross!).  After that I'll have
a good feel for the protocol.  Note that these new Iteration objects
(Loop objects? Iterators? unbound?) are MUCH easier to understand and
explain because they parallel normal looping structures.

dean