Adjust to `trio`'s strict eg nurseries throughout!

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.

piker/ui/_app.py

@@ -21,6 +21,7 @@ Main app startup and run.
 from functools import partial
 from types import ModuleType
 
+import tractor
 import trio
 
 from piker.ui.qt import (
@@ -116,6 +117,7 @@ async def _async_main(
         needed_brokermods[brokername] = brokers[brokername]
 
     async with (
+        tractor.trionics.collapse_eg(),
         trio.open_nursery() as root_n,
     ):
         # set root nursery and task stack for spawning other charts/feeds