Implement SimpleSmt::set_subtree

[plafer]

Closes #220

Note that set_subtree() returns Result<(), MerkleError> instead of Result<Word, MerkleError>, because I didn't know which value to return. The hash of the internal node where the subtree root was inserted? Is this relevant information?

I would also suggest we change the interface to

pub fn set_subtree(&mut self, index: NodeIndex, subtree: SimpleSmt) -> Result<(), MerkleError>

This would result in caller code that's easier to read (since the depth of the insertion is determined at the call site). I believe callers should always know the depth at which they want to insert, such as in miden-node where we want to insert a subtree of depth 13 at depth 8.

[bobbinth]

Looks good! Thank you!

Note that set_subtree() returns Result<(), MerkleError> instead of Result<(), MerkleError>, because I didn't know which value to return.

I was thinking we'd return the new root.

I would also suggest we change the interface to

pub fn set_subtree(&mut self, index: NodeIndex, subtree: SimpleSmt) -> Result<(), MerkleError>

This would result in caller code that's easier to read (since the depth of the insertion is determined at the call site). I believe callers should always know the depth at which they want to insert, such as in miden-node where we want to insert a subtree of depth 13 at depth 8.

I don't mind this change, but I do think that having to construct NodeIndex might complicate the interface a bit. But not a strong opinion by any means.

[sonarqubecloud]

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[plafer]

I was thinking we'd return the new root.

Changed it return the root (as an RpoDigest since this is how we represent the root elsewhere).