Import Automerge into Beehive

Beehive will need to know how to walk hash graphs across Automerge (AM) and Beehive (BH) ops. We may want to put this behind a trait eventually, but to get started we need to add AM to BH, and include the AM op history in the BH documents plus some methods to walk and/or collect transitive links so that BH can build the op visibility extents for AM.

In other words, this issue sets up the groundwork for us to do this calculation described in Patchwork (but not linked here because Patchwork is not access controlled prior to BH being done)

UPDATE: we may actually leave this up to Automerge

Start by reviewing how Beehive documents represent operation history and how Automerge hash graphs expose links. Resolve whether the groundwork belongs in Beehive or Automerge, then define the transitive-link traversal needed to build operation visibility extents; done when the ownership and required interfaces are settled and supported by tests.

Written by the indexing model from the issue text.