Use list* match pattern.

Because `list*` patterns faster than `list` on older versions of Racket.
Add `racket-backward-up-list'.

Like `backward-up-list' but works when point is in a string literal.
Improve a comment.
Fix tiny typo in README.md.
Add missing require to get clean byte compile.

cf commit cd5fd2e
Change company-mode initialization. Add racket-use-company-mode.

In my testing so far, this now works both when racket-use-company-mode
is:

- t when the buffer is opened.

- nil when the buffer is opened, and M-x company-mode is used manually.