Proposal: Make `Link` and `Block` interfaces compatible

As things stand today it is impossible to satisfy both Link and Block interfaces with a same instance, because both they collide on bytes field:

https://github.com/multiformats/js-multiformats/blob/15c52c8a5384e1bc84cccfa34ed6c7a78eb23978/src/block/interface.ts#L47-L55

https://github.com/multiformats/js-multiformats/blob/15c52c8a5384e1bc84cccfa34ed6c7a78eb23978/src/link/interface.ts#L21-L42

This is unfortunate because we're finding ourselves in a position where we would like to satisfy both interfaces along with following DAG interface:

interface DAG {
   links(): IterableIterator<Link|Block>
}

More broadly speaking we would like an ability to give a link with it's data attached (when we have it).