benchmarks: some additions to the reporting scripts

[eeckstein]

• add an option to the bench_code_size script to separately report code size for the swift libraries
• add an option to the compare_perf_tests script to output improvements and regressions in an single table.

[eeckstein]

@swift-ci smoke test and merge

[palimondo]

@eeckstein What is the intended use of single-table option? It looks like it is not used by CI bots, yet (recent benchmark report). Right?

I had a minor conflict around this change and though I might also write a test for it, while I'm at it...

[eeckstein]

I'd like to improve the output format of the benchmark report. But it's still in experimental phase. So it's not yet used by the bots

I had a minor conflict around this change and though I might also write a test for it, while I'm at it...

That would be cool!

[palimondo]

I was asking, because it isn't clear to me what it should do… (it also isn't using the formatter correctly)

Can you hold on changing this for a moment? I'm nearing completion on my PR and like to finish it without more conflicts. I promise to get back to this and change it the way you'd like to to work. Could you describe what you're after in more details?

[eeckstein]

Let me get back to you next week.