dat-core v5 API

Nobody has claimed this yet.

we wanna strip down the dat-core API to make it simpler/reduce scope.

here's the proposed new dat-core features:
- replication
- change processor hooks
- low level graph data manipulation api (similar to hyperlog api)
- graph external data pointers (with hashes pointing to commits and blobs etc)
- encoding support (abstract-encoder)
- core should be pure JS (works in browser and everywhere)

the goal is to be able to use dat-core in a situation where we might currently use hyperlog, e.g. https://github.com/moose-team/thanks/blob/fcfac4f7f4581f0fa0fecfe90f2122445da22ff4/index.js. dat-core is currently too heavy for these kinds of custom use cases. being able to use it for hyperlog-style things will let us build custom things while still being compatible with the entire dat high level ecosystem (like replication etc)

this means we would remove all other existing functionality currently in this repo and make new modules that the dat cli can use directly (tentative names):
- dat-fs (for blobs)
- dat-datasets (for tables and key/value storage)
- dat-containers (for VMs)
- dat-blob-resolver
- dat-commit-resolver

the dat cli won't necessarily change because of any of this, this is just a technical refactoring

No specific files or tests are named. Start by reviewing the current dat-core modules and the proposed dat-fs, dat-datasets, dat-containers, dat-blob-resolver, and dat-commit-resolver boundaries, then compare how the dat CLI uses them. Done would require an agreed smaller API and a refactoring that preserves replication and compatibility with the dat ecosystem.

Written by the indexing model from the issue text.