I forgot to push this change a year ago.

The cfg is valid as it's intended to be passed manually to rustc.

Fixes #21.

No actual code changes are necessary.

To be tagged and published to Cargo once merged.

I don't think the changes really deserve individual changelog entries.

- Stop using ATiltedTree/setup-rust@v1 because it apparently no longer
  exists.  The runners have rustup installed by default, so use that.

- Remove the separate `cargo check` job, since `cargo test` should
  be a superset of it.

- Add a test on nightly with default features.

- Add a test on MSRV (1.46) with default features.

- Add `-Dwarnings` to all jobs so that the presence of any warnings
  causes the job to fail.

  For the MSRV job, this requires allowlisting an `unknown_lints`
  warning that's known to occur there.

  Note that newer Rust versions may cause existing code to start
  producing warnings, so there is no guarantee in general that this
  crate compiles without warnings.  But it still makes sense to have
  this check in CI.  If someone is making changes to this crate, they
  should fix any warnings that have cropped up.

- Add a run of `cargo clippy` to the stable, stable-no-default-features,
  and nightly jobs.

  Probably not useful for MSRV.

- Add a build of all fuzz targets to the stable and nightly jobs.

  Running them would probably be too expensive.

- Fix a warning in one of the fuzz targets.

- Update `fuzz/ Cargo.lock` (this happened automatically when I built
  the fuzz targets).