[COMMIT] Stack rework

Heads up for internal hackers,

General stacks and control stacks are no longer circular. They are now
terminated at both ends with NULL pointers.

A "stack" now points to the top of the stack, not the base.

This simplified stack handling as well as the GC tracing, but
in one case, stack_entry() with a negative offset, results in
a linear walk. Is this even used?

The other benefit is we can do stack trees, which may make COW
and continuations less expensive. We can do COW per-chunk, rather
than per-stack. (currently unfinished)

We may need a "stack" object that will allow better random access
to the stack, rather than passing around the base or top.

-Melvin

• [COMMIT] Stack rework by Melvin Smith