Track compile times

Problem

Compile times are something we care about, but we're not measuring them systematically, so we have little visibility over them.

Potential solution

Building the benchmarks is a fairly good analogue for real-world usage where you have a warm compiler cache.

• Run benchmark builds with --timings.
• Parse the HTML report to get total compile time for each benchmark.
• Either:
  1. Make a comment on a PR if they improve / regress. or
  2. Upload them to CodSpeed (see last para of #5).
• Bonus points: Display the graph from the timings report in a comment on any PR for which compile times get significantly better/worse.
• Loads of bonus points: Convert the graph from timings report into an .out file so the graph appears as a faux flamegraph on CodSpeed. That'd give you a before/after diff too. (This is probably way too much work, and not worthwhile).

Start by examining how benchmark builds are run and how the --timings HTML report is produced. Review the CodSpeed option referenced in #5, then define whether the first milestone is PR comments or CodSpeed uploads; done means benchmark compile times are collected and reported for regressions or improvements.