Improve GitHub comments.

- Remove linebreak in table labels, which makes the table shorter and
  easier to read.
- Use "-" instead of "N/A", because it's visually smaller.
- Use ❌ and ✅ instead of 😿 and 🎉, because they're clearer, in shape,
  colour, and meaning.
- Change "max" column to "range", which is more useful.
- Reorder columns, to put more fundamental things first.
- Change meaning of the "All" rows to mean all results, not just
  relevant ones. Good for gauging the overall effect, especially when
  only a small number of benchmarks are affected. Also add an "All" row
  for secondary benchmarks.
- Reorder rows. "All" first because its the biggest one. And
  improvements before regressions, to be optimistic.
- Tweak alignments in the table.

Author: nnethercote