Opam 2 only supports md5, sha256 and sha512. We don't need the extra
complexity of other hash algorithms. Opam 2 will fail to parse such an
opam file anyway.
- Tar.3.0.0 was released! So we can drop the pin and depend on it
directly.
- Use digestif in favor of the bygone mirage-crypto hash
- Move most of the hashing logic into archive_checksum.ml
- When checking the checksum we should check the checksum chosen, too. I
believe this was a bug that was hidden due to opam packages always
using one of md5, sha256 or sha512 for checksums.
