Unlike links, these are checked by sphinx and it complains if they
don't match.
Also they have a better chance of doing something useful in outputs
other than html.
It was always a bit ridiculous that argparse required `X-longflag` if
that "X" short flag was never actually used anywhere.
Since the short letter is for getopt's benefit, we can hack around
this with our old friend: Unicode Private Use Areas.
We have a counter, starting at 0xE000 and going to 0xF8FF, that counts
up for all options that don't have a short flag and provides one. This
gives us up to 6400 long-only options.
6.4K should be enough for everybody.
This was written before local-exported variables did anything useful.
Passing these vars as local-exports removes the need to define the
validation function with `--no-scope-shadowing` which is quite the
hack.
