Hook up icount benchmarks to CI

[aochagavia]

This PR adds a GitHub actions job that does the following:

• Benchmark the PR's target branch and store the results in a temp file.
• Benchmark the PR itself and store the results in a temp file.
• Compare the two runs and set the job summary to display the pretty-printed results.
• Let the job fail if there is a noteworthy difference in instruction counts, hinting you should look at the job summary to see the details.

You can already see it in action by looking at the new job triggered by this very PR 😉. It is failing, because the icounts have increased a lot (I discovered we need to run the resumed handshakes about 30x to filter out noise, instead of 3x).

Relevant context:

• It's not possible to post the benchmark results as a comment to the PR directly from the GitHub Actions job. This happens because the job runner has read-only access to the issues when the PR has been created from a fork (see the docs). Codecov manages to do this by running as a separate application, with its own permissions.
• I'm using the term "noteworthy" instead of "significant" to avoid giving the impression we are doing anything fancy with statistics here.

[codecov]

Codecov Report

Merging #1431 (71633e6) into main (676df24) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1431   +/-   ##
=======================================
  Coverage   96.49%   96.49%           
=======================================
  Files          71       71           
  Lines       15169    15169           
=======================================
  Hits        14637    14637           
  Misses        532      532           

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[cpu]

Looks good, just one nit to flag.

[cpu]

I think we can go ahead and merge this and iterate if there's any additional feedback.

[aochagavia]

Thanks for the reviews!