Implement `MastForest` merging

[PhilippGackstatter]

Describe your changes

Closes #1529

Implements MastForest merging.

The implementation is explained in comments and it's hopefully sufficient to understand the PR, which is why I'm not adding the details here again. If any explanation is insufficient I'm happy to expand on it!

Couple of notes and ideas:

• The MastForestDfsIter could be 1) made public and 2) could be made more general to allow for in-order, pre-order as well as post-order iteration of MastForests. Let me know if any of these are desirable. For now it seems best to keep them private.
• The MastForestMerger is also kept private because the useful functionality can be exposed through the methods added on MastForest directly and there is no benefit in terms of functionality by making it public.
• Moved EqHash into miden-core.
  • One annoyance is that this type needs to be public in miden-core so that it can be used in miden-assembly by MastForestBuilder where it originally lived. Moving MastForestBuilder into core doesn't seem fitting since it is used only for assembly and specially made for that. Overall it's not bad to have EqHash public in core.
  • Because that type needs to become public, I would propose to rename it to something a more easily understandable, like MastNodeFingerprint and perhaps introduce pub type DecoratorFingerprint = Blake3Digest<32> for decorators. I think the "fingerprint" nicely describes that this type can be used to compare mast nodes although I'm of course open to other suggestions.
  • If this type becomes public we should document its public functions (added TODO).
  • This topic could overall be handled in a quick follow-up PR if desired.
• I would like to test the implementation with some more complex "real-world" input, but this is difficult in miden-core without an Assembler. I don't want to move those test into miden-assembly though as that separates the implementation and the tests crate-wise. Even with an assembler, it would still be somewhat challenging to come up with the expected merged forest to assert against without basically handwriting it. Although I suppose asserting a couple of properties might be sufficient instead of the entire forest, so it would still be an improvement. One possibility I see to add test in miden-core would be to generate snapshots in miden-assembly, for example, and add them as plaintext serialized MastForests to miden-core. Snapshot testing could be done with something like insta or without, I suppose. This would require adding #[cfg_attr(test, derive(Serialize, Deserialize))] to MastForest and all its descendants which is fine I guess. I'm happy to receive some general input on the testing story for this task though.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.

[bobbinth]

I would like to test the implementation with some more complex "real-world" input, but this is difficult in miden-core without an Assembler. I don't want to move those test into miden-assembly though as that separates the implementation and the tests crate-wise.

I actually think it may be fine to have some tests for merging forests in the miden-assembly crate. It would simplify things a bit and there is not too much downside.

[PhilippGackstatter]

I updated the implementation to use the desired merging behavior for external nodes with decorators present. The refactor to support this turned out to be fairly complex but I think it's alright now. I still need to add documentation in many places, handle some errors and probably some other small things, but conceptually I think it is in a reviewable state now again.

[PhilippGackstatter]

I implemented the refactoring now, but there are still some things left to clean up for sure. I added an assertion to most tests now that assert that the "child id < parent id" property holds, so that's nice :tada:. If you have feedback on the approach now, I'd be happy to hear it and the other nits I will address tomorrow.

Also, sorry for the huge code changes, hopefully that was the last one.

[PhilippGackstatter]

After adding a test that used the MastForest from the Miden StdLib I encountered a small bug in the iterator logic, but that is fixed now. The PR is now ready for a full review again.