Subtree consistency proof verification

[phbnf]

Towards #225.

This PR introduces VerifySubtreeConsistency. The implementation is a bit different from the current VerifyConsistency implementation, hence the PR length. However I believe that this one is easier to follow, and closer to specs:

• First we reconstruct the original and the grown subtree roots
• Then, we treat the rest of the proof as an inclusion proof

This is how I've explained things in the design doc as well.

I've got two other candidate implementations, that are more similar to the current one, but I find them quite hard to follow. The complexity is around figuring out which proof nodes we need to recompute the subtree root hash. For regular consistency proofs, breaking the proof down where the path of |end-1| and |size-1| is enough. It's a bit more complex when it comes to subtrees:

• This implementation uses a decompSubtreeProof function, but I still find quite it hard to follow.
• That implementation uses making, which isn't that straight forward at first sight.

I ran a few benchmarks, and did not find any significant performance difference between these implementations.

I've also gone maximalist on comments, and I'm happy to remove some.
I'm sure that either implementation could be improved, but since I'd rather that we agree on a direction first.

[codecov]

Codecov Report

❌ Patch coverage is 55.17241% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.17%. Comparing base (0845efa) to head (69c0e7f).

Files with missing lines	Patch %	Lines
proof/verify.go	55.17%	25 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
- Coverage   49.81%   49.17%   -0.65%     
==========================================
  Files           8        8              
  Lines         807      844      +37     
==========================================
+ Hits          402      415      +13     
- Misses        397      421      +24     
  Partials        8        8              

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