Change string representation of hashes

[rgrinberg]

The wiki says the following:

If you find a hash that uses colon as a separator (:), don't use that. This relies on undocumented behavior and is not officially supported.

Should opam2nix switch to using dashes perhaps?

[timbertson]

Probably. But SRI hashes aren't implemented in nix-hash, you need nix which must first be enabled as an experimental feature. I could implement the SRI encoding myself, but it's not standard base64 so I'll just wait it out 🤷