Using `tractor.trionics.collapse_eg()` as needed to avoid, at the least,
crash-worthy (in debug-mode REPL-ing terms) nested cancellation egs that
exhibit on SIGINT/ctl-c of each "app" (chart & daemon).
Also a bit of renaming of all `trio.Nursery`s to `tn`, the new "task
nursery" shorthand-var-name being used in all our other `tractor`
related projects.