Zstd compression can be enabled by providing `-zstd` or `-zstd-level <level>`
to `init`, `add`, or `backup`. With `-zstd` the compression level will be
`default`, and with `-zstd-level` the level can be any of `fastest`, `default`,
`better`, or `best`.
highwayhash 1.0.1 contains a bug leading to incorrect hashes on arm64 machines.
The 1.0.1 version is retained in github.com/gilbertchen/highwayhash so the hash
can be checked again if a mismatch is detected by 1.0.2.