Implement Fugue and Sync9

[tantaman]

Background research is here: #65

With a Fugue implementation sketched at the bottom -- #65 (comment)

Sync9 -- https://github.com/braid-org/braidjs/tree/master/sync9 & https://github.com/pkulchenko/syncX

Sync9 is interesting given that it stores actual character indices in the CRDT so appending new writes does not require any lookups of parent events and constructing a doc from the event log is trivial. Sync9 also solves the same interleaving problems as Fugue.