Use the `digest` prefix on all problem types

[ioggstream]

Question

• Since these problem types only apply to Digest, maybe we could prefix them with digest-.
• This will prevent clashes on generic statements such as "unsupported algorithm", that may apply to different specs.

New names could be:

• digest-unsupported-algorithm
• digest-invalid-value
• digest-mismatching-value

If we want to retain current names, we could at least have unsupported-digest-algorithm.

[Acconut]

That's a good idea and makes the naming more consistent. I'll open a PR for this.