shell: show screen state during evaluation

All highly experimental. Current constraints:

* No tail recursion elimination
* No heap reuse
* Keep implementation simple

So it's slow, and I don't want to complicate it to speed it up. So I'm
investing in affordances to help deal with the slowness. However, in the
process I've taken the clean abstraction of a trace ("all you need to do
is add to the trace") and bolted on call counts and debug-prints as independent
mechanisms.

0 files changed, 0 insertions, 0 deletions