Optimize the API

Optimize the API

- Before, if an add-on called `vimfx.set()` around 200 times it could delay
  Firefox's startup time with almost a second. Now there's no delay at all.

- Marks internal variables in public.coffee with a leading underscore. There's
  nothing stopping people from importing them, but no they at least look
  private.

- Remove `vimfx.refresh()`. That is done automatically now, in an efficient way.

- This also makes the test's teardown more robust.