Remove dependency on multihashing module

[yusefnapora]

Since the synchronous multihashing module is no longer recommended, this removes our dependency on it.

We're were using it in the b58MultihashForBuffer helper (and Datastore.put which was written before the helper and can use it instead).

We are calling that helper in a couple of sync methods (like the constructor for ExpandedSimpleStatementBody), so I did the lazy thing and just did a synchronous implementation using node's crypto.createHash instead of trying to make everything async so we can use https://github.com/multiformats/js-multihashing-async which uses crypto.createHash on node anyway.

[parkan]

👍