Resending a revised patch again. I find it easier to just reformat
against master, and I apologize if there is some other workflow that is
preferred.

In testing, I found one more area for improvement: we should try to use
precomposed characters when they exist instead of strings containing
base letters and combining characters. That meant doing away the the
automatic concatenation of vowels and combining diacritics permutations
in favor of just specifying all permutations manually (like most Quail
packages do). That is more error-prone, but I've double checked and
tested it and it looks good.