[Task]: Create nullifier tree wrapper over Smt

[polydez]

Node's components affected by this task

• Block producer

What should be done?

From @bobbinth:

While I think 0xPolygonMiden/crypto#288 could be independently interesting, I wonder if we should use a different approach for this specific issue.
This should help abstract away the logic of how block numbers are associated with nullifiers and would leave more flexibility for the future (e.g., we could build a specialized implementation of the NullifierTree which assumes that leaves are just 32-bit values).

How should it be done?

Specifically, we could create a new type NullifierDb or NullifierTree which would look something like this:

pub struct NullifierTree(Smt);

impl NullifierTree {
    pub add(&mut self, nullifier: Nullifier, block_num: u32)-> Result<(), NullifierTreeError> {
        // returns an error if the nullifier is already in the tree
    }

    pub fn get_block_num(&self, nullifier: &Nullifier) -> Option<u32> {
        // returns None if the nullifier is not in the tree
    }
}

When is this task done?

When wrapper with requested functionality is implemented and all components of Miden Node use it for nullifier's tree instead of bare Smt with conversions of nullifier's data.

Additional context

More context here: #266