Fix key map.

- Use `define-derived-mode` AFTER `defvar`-ing the mode's keymap.
  Otherwise, although there's no error message, the mode doesn't use the
  keymap -- not until the `defvar` is evaluated again.

  (I guess I still don't have a crisp understanding of Elisp load/eval
  model, vs. Racket's module system. It's also interesting that the Run
  command avoids exactly this sort of problem -- where interactively
  working at the top-level can leave you in a state that loading the
  file will NOT recreate. Which is how I didn't notice this issue, until
  I restarted Emacs.)

- Parent `racket-mode-map` to `lisp-mode-shared-map`.

- Add C-x ` "Next Error" command to menu. Because discoverable.