Redesign multihash::Error

[aatifsyed]

Do we have other const functions that might be affected by the same problem? I wonder what a long-term solution could be here.

In the issue, you said that the problem is the io::Error in Kind? I tried previously to remove that one because it only appears in certain functions. Perhaps that is worth exploring because most functions don't actually return an io::Error.

If you'd like my opinion, you should impl From<YourErrorType> for std::io::Error, and have io functions return an io::Result, which may be InvalidData with a source = Some(YourErrorType), where YourErrorType is a slimmer Error

(This would be a breaking change.)
((I think that's fine, you're pre 1.0.0, and the API should be allowed to evolve past current limitations))

Originally posted by @aatifsyed in #331 (comment)