This should be a true "release candidate" in that the only change we
expect in a final release is a bump to `cpufeatures` v0.3.

This release includes `--cfg chacha20_backend="..."` changes in #520, as
well as the opt-in AVX-512 backend that can be enabled with `--cfg
chacha20_avx512`, and also bumps the `cipher` crate to v0.5, the stable
release.