The benchmark action's "cargo" tooling assumes the standard bencher
output format, and can't parse criterion's output. Fortunately,
criterion can also provide bencher output for compatibility.
In order to pass criterion flags via `cargo bench`, we need to ensure
that all benchmarks use it (otherwise the flags are rejected by the
built-in benchmark harness as unknown). Since we don't actually have any
benchmarks inside the library itself, we simply disable its benchmark
harness.
The workflow will comment on commits that cause a performance regression
of at least 200% (e.g. proving taking twice as long); we'll tune this as
we figure out how well benchmarking works on standard GitHub builders.
Jack Grigg