State-based merging

[mweidner037]

Completes https://github.com/composablesys/collabs/issues/239

Rewrites CRuntime.load and all CRDTs' load functions to allow loading at any time. If there is already a non-trivial state, the loaded state is merged with that one, in the style of state-based CRDTs.

Additionally, load functions now emit events describing their changes (including incremental changes during merging). This eliminates the need for special cases to refresh the display after loading.

I've also made minor API changes and rewritten the list CRDT internals.

Still TODO:

• [x] - Update packages besides core and crdts
• [x] - Update demos
• [x] - Unit tests for merging, new list internals

[mweidner037]

Minor API changes:

• Add Position type (alias for string) to IList.
• Remove info in CRuntime.transact. Instead, apps should use their own CMessenger.
• CRuntime events: remove Load, instead emitting Transaction. Also remove isLoaded.
• CRDT meta: replace vectorClockGet with vectorClock interface; in Collab.load, use type CRDTSavedStateMeta instead of CRDTMessageMeta.
• Remove option to return null from Collab.save. Also, add option to pass null to Collab.load, but with a different meaning from before (tied to Collab.canGC).

Behavior changes:

• CSet.add is now a pure CRDT operation, hence can be used with RunLocallyLayer.
• CSet.fromID will remember deleted values (hence return non-undefined) until the end of the current Transaction. This avoids surprises where you try to reference a just-deleted value in an event handler (e.g., see the comments in CMap's internal event handlers).
• Deduplicate messages in CausalMessageBuffer.

Other:

• Collab tests for most concurrent ops, merging, and events.
• Add Istanbul code coverage checker to the crdts package. Run with npm run coverage.