Tooling for unparsable `Multiaddr`

[mxinden]

Sample use-case:

• Roll out of new Protocol::X.
• Old DHT nodes should forward Multiaddrs with Protocol::X of new nodes even though they can't parse the Multiaddr. Currently they don't.

Considerations:

• Instead of providing e.g. an Unparsable type in multiaddr, each user that cares could also carry a Either<Multiaddr, Vec<u8>>.
• One could add a Protocol::Unparsable(Vec<u8>) containing the remaining unparsable bytes. This might break existing implementations as they depend on Multiaddr either to succeed or fail, but not fail partially.

Related:

• https://github.com/multiformats/multiaddr/issues/155