Refactor signing keys, Publisher Id into classes

[yusefnapora]

This changes the PublisherId type into a js class instead of just a flow object literal type, and adds some handy sign and verify methods to it, as well as static load and save to persist to disk. It also wraps the callback-based libp2p-crypto key interfaces with PublicSigningKey and PrivateSigningKey classes that use a Promise interface instead.

Also adds a test for loading a publisher id from concat and using it to sign and verify a message.

[parkan]

hooray