packages/gossiplog - add branches to gossiplog

[rjwebb]

This is part of #206

This PR adds a branch value to GossipLog which lets us implement a more efficient version of isAncestor by allowing us to traverse the causal log's branches instead of visiting each message one at a time.

To do:

• Add a child index, this is the inverse of AncestorIndex
• When a message is applied in the GossipLog, compute the "branch" based on its parent messages
• Update a table of where branches merge into each other ("branch merges")

How has this been tested?

Unit tests

Branch values:

• For a sequential chain of messages assert that the branch value is 0
• For a set of unconnected messages (i.e. none of them share a parent) assert that the branch values are all different
• For branching and merging messages test a variety of scenarios

Branch merges:

• Add assertions to the branch values tests but for the branch merges table as well

Regressions:

• CI tests pass
• Tested with example-chat (including login, all signers, and exchanging messages)
• Tested with @canvas-js/test-network: (optional)

Does this contain any breaking changes to external interfaces?

• Contract interfaces
• Core interface
• CLI
• Data storage formats, including IndexedDB, SQLite, or filesystem storage (will this break existing apps?)