In the same spirit as multiformats/rust-multiaddr#71, I want to raise awareness about how disruptive the breaking changes in this fundamental library are for the ecosystem.

I have a few suggestions:

• Error could be made non_exhaustive or completely opaque. The latter would be my personal preference as it gives you more flexibility as a library author.
• Move MultihashDigest & Code to a separate crate. All we need in rust-multiaddr is the Multihash type which acts as a type-safe representation for a multihash. The machinery around MultihashDigest, the actual hash implementations etc could all live in a separate crate. This would reduce the API surface drastically and allow multihash to remain more stable (and almost dependency free)
• Code uses conditional enum variants. That is not ideal. Features are additive across a dependency tree in Rust. Conditional enum variants can turn an exhaustive match in a library into a non-exhaustive one simply because the end-user activates more features of multihash.

Curious to hear your thoughts. (And happy to send PRs if we agree on something!)