Add wf-projection-stress-65510 benchmark.

[eddyb]

This is a tweaked version of the stress test from rust-lang/rust#65510.

The version in this PR should take about a couple seconds in cargo check (I was getting 2.24s locally, using nightly) before any fix to rust-lang/rust#65510 is attempted, and prevent regressions once a fix does land (as well as letting us measure a fix in the first place).

[nnethercote]

Thanks for updating the README file!

My only complaint is that the benchmark name is very long: 5 characters longer than unicode_normalization, which is currently the longest name. How about wf-projection-stress or wf-stress-65510?

[eddyb]

Oh, whoops, didn't consider that (I assume it's relevant to the UI?).
At the very least, projection could be assoc or proj easily.

@Mark-Simulacrum Can the little data already collected be renamed to avoid the longer name showing up anywhere at all?

[nnethercote]

We already have issue-46449 and regression-31157 benchmarks, too, so those are patterns that could be followed.

[Mark-Simulacrum]

Yes, whatever data we have can be renamed; it's a bit of work but not too bad. I'd appreciate a PR doing the rename itself though (moving the directories over and such), I can do the legwork of renaming older data myself right as I merge.