Improve GitHub comments.

- Use lists instead of tables, which cause lots of layout problems.
- Show "range" instead of just "max".
- Include mean and range of all results, as well as those numbers for
  the improvements and the regressions.
- Put all the text at the top.
- Make clearer the overall ok/not-ok evaluation, and which metrics
  contributed to that (i.e. instructions, yes; RSS and cycles, no).
- Use ❌ and ✅ instead of 😿 and 🎉, because they're clearer, in shape,
- Include a link to a document with some explanation of how to evaluate
  the overall result, for the rare cases where it is misleading or wrong.
- CC `@rust-lang/wg-compiler-performance` on regressions that occur on
  master.

Author: nnethercote