All we need from the multihash is for it to be a data structure that we pass around. We only ever use the identity "hasher" and the sha256 hasher. Those are easily implemented without depending the (fairly heavy) machinery in `multihash`.
Unfortunately, this patch by itself does not yet lighten our dependency tree because `multiaddr` activates those features unconditionally. I opened a companion PR for this: https://github.com/multiformats/rust-multiaddr/pull/77.
https://github.com/multiformats/rust-multiaddr/pull/77 is another breaking change and we are trying to delay those at the moment. However, it will (hopefully) land eventually which should then be much easier to implement.
Fixes#3276.
Pull-Request: #3514.
