Refactor core, crdts, collabs

[mweidner037]

Refactor of core, crdts, and collabs packages to simplify the library and (hopefully) reduce learning curve.

Changes

Major

• Simplify naming scheme for Collabs: the main CRDTs now have names like "CVar" or "CValueSet". Alternate semantics are either options on the main CRDTs or deleted.
• Reduce "C*" names to avoid cluttering Collabs. (Interfaces -> I, e.g., CMap -> IMap; events & events records drop the C, e.g., CSetEvent -> SetEvent.)
• In the collabs package, only re-export the "commonly-used" exports. For advanced use cases, import other stuff from core or crdts directly.
• Rename CObject.addChild -> CObject.registerCollab, so that it is the same as CRuntime.
• AbstractDoc that you can extend to wrap your CRuntime and top-level Collabs. This lets you carry them around in one var, like a Y.Doc.
• Replace "batching" with "transactions" (CRuntime.transact method, CRuntime "Transaction" event).
• Remove context and "generic" MessageMeta, which I found confusing & not very type seafe. Instead, there is UpdateMeta; its runtimeExtra property can be extended by the Runtime, but not by other ancestors.
• CollabIDs: instead of make/get from CollabID class, use Parent.fromID and Parent.idOf (which replace getNamePath / getDescendant). Also, they're now JSON-serializable.
• Make IList "positions" more useful, and remove the CCursor wrapper (instead, use positions directly).

Minor

• Remove less useful serializers.
• Namespace some utility functions with static classes or singletons.
• Change load/save type - instead of serializing all children immediately, parents can pass this responsibility to the Runtime (see SavedStateTreeSerializer).
• Add CConst.
• Redo AbstractSet, etc. in a way that appears to be working type-wise. (Basically: give up on TS mixins, just use JS mixins and type them manually.)
• Add Collab.finalize method. This avoids needing every CSet user to expose a "ValueDelete" event.
• Rewrite CRuntime. It is now a simplified "flat" implementation, instead of several layers with confusing interactions.
• Rewrite CText to use a CValueList internally, instead of re-implementing it.
• Move the benchmark text trace into a JSON file instead of JS, so Github doesn't show us as a "majority JS" project.
• Misc cleanup (e.g., all Collabs take an options object instead of misc optional constructor params).

Todo

• Check for other minor tasks/issues that we can work in here
• Fix demos, benchmarks broken by the changes
• Update docs to match (doc comments & .md files)
• Preview Typedoc

[mweidner037]

The docs are partially updated (e.g. straight name changes) but I have not checked them; will do so in a minor update.