MST implementation with support for operation inversion

[bnewbold]

This is fairly similar to indigo/mst, but written from scratch in a more go-idiomatic style, with the intent of supporting record op inversion (for the "inductive firehose" proposal).

Goal is to have something that passes full interop tests with TypeScript codebase, and can validate commits on the firehose. Would take more perf work after that, and probably review, to get this in a state where it could be in the hot path in prod.

[bnewbold]

Interesting: tests are passing for me locally (including clearing test caches), but failing in CI. Will need to dig in to that.

An open design question is how to split "repo" code from "MST" code. Most of this comes down to managing records to/from CAR slices, repo exports, blockstores, etc. And concept of "record operations": is that an MST concept, or a repo concept? Is testing inversion of ops something that happens as a batch from repo code, or by MST code?

Having helpers which "walk" the MST in various situations could help: when parsing blocks into a tree (visiting each node in-order); when serializing blocks to CBOR (either all blocks as repo export, or just "dirty" blocks); or just generally walking a subset or the entire tree.