Add racket-util.el

- Move `racket--symbol-at-point-or-prompt' there.

- Add `racket--trace`.
Move provide to EOF.
Add racket-find-collection, racket-open-require-path. Closes #67.
Make racket-open-require-path faster. Issue #67.

Use a more-direct approach so that it keeps up with fast typing and we
don't need all the timer hoo-hah as a poor work-around.
Automatically start REPL when needed.