Implement error codes for the mtree_verify instructions

[Overcastan]

This PR implements the ability to specify error codes for mtree_verify instruction the same way it is done for assert instructions.

TODO:

• update changelog

Related issue: #1264

[bitwalker]

Looks good! Just one small nit, but your call on that one.

[bitwalker]

Nit: I would name this something a bit more precise, perhaps InstWithErrorCode? Most other instances of Maybe* are macros which can be applied to a non-terminal, but that isn't the case here - the others are more precise about what is being parsed, so less ambiguous at first glance.

[bobbinth]

Looks good! Thank you!

One outstanding thing, probably for a different PR, is figuring out how we can treat MerklePathVerificationFailed error in a similar way as FailedAssertion. Specifically, for FailedAssertion, we have a dedicated host handler which can process assertion errors in a custom way (i.e., by mapping them to user-friendly error messages). Should we do something similar for the MerklePathVerificationFailed?

If we do add a special handler to the Host for MerklePathVerificationFailed - should it cover only mtree_verify instruction, or other Merkle-tree related instructions as well?

As I mentioned, this is probably for a different PR - but I think we should have a good understanding of what we want to do before merging this PR.

[Overcastan]

We can add a Host handler for Merkle tree errors exactly the same as we do it for Assert now, with handler like on_merkle_tree_error. I think it makes sense to use it for all Merkle-tree related instructions: for now we have only mtree_verify, but probably other instructions eventually will have error codes as well.

By the way, I noticed that we call on_assert_failed Host function only for Assert(err) operation, but not for U32assert2(err). Formally it is also an assert operation, although it is used in a slightly different way. Should we leave it as it is?