Fix interleaving of comments imperative structures.

[jordwalke]

Fix interleaving of comments imperative structures.

[jordwalke]

Fixes #1767 (weird unit formatting) and (makes formatting idempotent #1768)
(as well as some other comment formatting improvements).

There's still quite a few cases where the output is ugly when including end of line comments, but this is progress. (Those cases weren't mentioned in those two Issues)

There primary remaining issue is just the case where you have exactly one operation inside for/while/if braces, but then have a final comment after it before the closing brace. If any braces have multiple statements in them, the formatting is fine. This example wasn't mentioned in the Issues.

if (trueThing) {
  /* Comment before print */
  print_newline();
  /* Comment after final print */
}

[jordwalke]

Merging! Let me know if this caused any problems, and we'll quickly revert.

[kyldvs]

Thanks :)