feat: atrium-repo

[DrChat]

This builds on str4d's work here: #168.

Big notes:

• Complete merkle search tree implementation (but still lacking fuzz tests)
• Complete Repository implementation with support for basic CRUD operations
• Added a new pair of traits for block storage: AsyncBlockStoreRead and AsyncBlockStoreWrite.
  • In-memory storage backend (primarily intended for testing)
  • CAR storage (supporting both reading and writing)
  • Users can implement other storage sources, like Azure-backed block storage.

Test coverage (cargo llvm-cov -p atrium-repo --html --open):

---

• Merkle Search Tree
  • Parse Node
    • Read Node from bytes
    • Verify depth and sort order
    • Limit the number of TreeEntrys per Node to a statistically unlikely maximum length.
    • Consider limiting the overall depth of the repo, or other parameters, to prevent more sophisticated key mining attacks.
  • Locate key within node
  • Locate keys within node with a given prefix
  • Add entry
  • Edit entry
  • Delete entry
• Repository
  • Load from a CAR file
  • Load from a firehose record
  • Read/write records inside of a repository
• Storage
  • CAR files
    • Reading CAR files
    • Verify completeness of the repository structure.
    • Robustness to both duplication and de-duplication of blocks.
    • Ignore any unnecessary or unlinked blocks.
  • Commits from firehose records

[DrChat]

I think the PR is in a pretty good state for a MVP.
My time is limited so I won't be able to quickly iterate on the functionality for creating new commits for a repository. Still planning on it, but the work here can be pushed up in the meantime :)

[erlend-sh]

Pinging @str4d since this builds on their prior work.

[sugyan]

I have written several comments on areas unrelated to the internal logic. There are also a couple of points from clippy that I hope you can correct as well.

[DrChat]

Note: It looks like there's a pretty exhaustive MST test suite here that could probably be leveraged to validate this implementation.
https://github.com/DavidBuchanan314/mst-test-suite

[sugyan]

I couldn't review the correctness of the implementation and other details, but I would like to merge them once. If @str4d finds any problems later, please let us know via issue or pull-request.

Thanks @DrChat for your great work!

[DrChat]

Thank you! And I'm sure there are some bugs in the implementation - I'm hoping that someone else will shake them out if I don't get around to it :)