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[Kobzol]

The performance summary tables in GH comments (e.g. this one) show all metrics other than instruction count in a collapsed manner. I think that this sometimes leads to missing interesting information in these tables, if no one clicks on them.

This PR adds a short summary of the performance result to the Results reveal button, so that it is clear how large is the effect even without clicking on the button. It should look like this:

[nnethercote]

tooltip seems like a strange name. To me that means something that shows up when you hover over a GUI element. Maybe call it summary? Otherwise, LGTM.

[Kobzol]

I also didn't like the name, but couldn't think of a better one. Summary is already kind of overloaded in the code, but it is a good name nevertheless. Changed the PR to use it.