Add CDF plot to benchmark reports

[bheisler]

CDF plots are less commonly understood, but once one learns to read them they're actually much better at conveying information than the PDF plots cargo-criterion generates. We should generate a CDF plot as well as the PDF plot for the more advanced users.

Could maybe generate a normal-quantile plot for the really advanced users as well?

[link2xt]

Related issue in criterion.rs: bheisler/criterion.rs#443

[link2xt]

See top plot at deltachat/deltachat-core-rust#2382 (comment) for an example of how misleading PDF plots can be. Red curve suggests some values fall below 300ms, but in fact there are no runs at all which completed in under 300ms.