The "principal" environment stack was the one that was associated with the
"principal" parser and would dispatch changes like to TZ, etc.
This was always very suspicious, as a global; now we can remove it.
These have clearer sync/unsync semantics and now ship with rust itself.
They don't paper over any possible cross-thread issues, and we can specifically
choose which we want for the purpose.
I think commit 8386088b3 (Update commandline state changes eagerly as well,
2024-04-11) broke the alt-s binding.
This is because we update the commandline state snapshot (which is consumed
by builtin commandline and others) only at key points. This seems like a
dubious optimization. With the new streamlined bind execution semantics,
this doesn't really work anymore; any shell command can run any number of
commands like "commandline -i foo" which should synchronize.
Do the simple thing of calculating the snapshot whenever needed.
