shell: reenable the trace

We now have a couple of protections:
  - if we get close to running out of space in the trace we drop in an
    error
  - if we run out of space in the trace we stop trying to append
  - if there are errors we cancel future evaluations

This is already much nicer. You can't do much on the Mu computer, but at
least it gracefully gives up and shows its limitations. On my computer
the Mu shell tries to run computations for about 20s before giving up.
That seems at the outer limit of what interactivity supports. If things
take too long, test smaller chunks.

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