Use hex literal.
Show error for undefined.
Don't bind C-c C-p to racket-cycle-paren-shapes in REPL.

That's already the binding for comint-previous-prompt.
Have describe write its HTML to a temporary file.

This is more resilient than writing it into the *Racket REPL Redirected
Output* buffer, because comint-redirect-send-command-to-process looks
for comint-prompt-regexp to detect end of output. If a line of HTML
starts with something that looks like that -- e.g. ">" or "p>" -- then
it will break. Attempting to make comint-prompt-regexp resilient to that
is a losing game. Instead, it's easy to write the HTML to a file, and
print to the REPL output simply a string that is the file pathname.
Relax comint-regexp-prompt.

I'd made it stricter when working on the problem that commit 0af8f55
instead solves more fundamentally.
Add key binding and menu item for racket-describe.
Show the "kind" and provided-by info.

Also, delete code for the now-unused plain text format output.