bug#35383: 27.0.50; Complete process of decoding Gnus group names

[Eric Abrahamsen]

This is part two and the completion of bug#33653, which changed Gnus's
obarrays into hash tables, and group names from symbols to (encoded)
strings. The commits in the recently-pushed "scratch/gnus-decoded"
branch change group names to decoded strings.

Bug 33653 was a mess for a few reasons: I partitioned the changes
poorly, didn't call for testers, and it turned out that I was locally
testing a mismash of that change plus a couple changes included in this
branch, which hid some bugs.

This time around I'll keep it cleaner: I'll locally test only this
change in isolation, I'm writing a semi-interactive test suite for Gnus,
and in a few weeks I'll rope in three or four users to road-test the
changes. The upside is that, once these changes are stabilized and put
in, it will eliminate a whole class of potential bugs.

In the meantime I'm hanging this here as a placeholder. If any brave
soul does decide to give it a test-run in the meantime, back up your
.newsrc.eld file first!

Eric