Replace Mermaid with HTML reporting

[zmitchell]

Mermaid is a quick and dirty way to get the kind of diagrams I'm looking for, but it has a number of limitations.

Ultimately I'd like something similar to the output of cargo build timings. Having perused the code it doesn't seem unreasonably difficult to just copy their templates and modify them for our needs. It would also allow us more flexibility when it comes to displaying information other than execution time e.g. you could color spans according to their process group.