Coherent number formatting

[art-w]

The PR benchmarks displays a comparison table with master that is very noisy, especially when no performance change is introduced:

Some ideas:

• Fix the floating point numbers to max 2 digits of precision (with no random e-14 scientific notation introducing misalignment... but at the same time, we still need to be able to display tiny metrics!)
• Make sure the font has a fixed size and that everything aligns nicely to ease reading (perhaps the css text-align: "." center mentioned in https://alistapart.com/article/web-typography-tables/ can be useful)
• Add units!
• Finally, it would be nice to reuse the work from WIP: Look for changed significantly metrics and notify authors #358 to only display the metrics that require further inspection, a big table of mostly 0.0001% hides the actually useful data.

(reported by @Leonidas-from-XIV from this example PR https://autumn.ocamllabs.io/ocaml-community/yojson/pull/152?worker=autumn&image=ocaml%2Fopam%3Adebian-11-ocaml-4.13 )