I did some research and experiments. For good or bad the `bind` command
requires the use of wide char codepoints (e.g., \u1234) for non-ASCII
chars. So don't force the use of the POSIX locale, but do provide it as
an option for people who want to see the individual bytes rather than a
decoded wide char.
Simplify the format of the information displayed for each character. There
really isn't much point in providing decimal, octal, and hexadecimal. Just
print hex and symbolic representations.
Add an example `bind` command that a user can copy/paste.
Closes#3183
Another developer noticed that redirecting stdin of `fish_key_reader`
results in weird behavior. Which is not at all surprising. So add checks
to ensure stdin and stdout are attached to a tty.
Add some rudimentary unit tests for this program.
