Replace mtree_cwm with mtree_clone

[grjte]

As discussed here in #750, SMT and MMR would be easier to support if we change the way that the AdviceProvider handles clone and update operations.

Currently we offer the copy with modify operation mtree_cwm, but we would like to remove this and add mtree_clone instead. Users who want to make an update in the AdviceProvider would instead then call mtree_clone followed by mtree_set.

The required changes as described by @bobbinth are as follows:

To support a separate clone operation, we need to add a new method to the AdviceProvider interface. Something like:

pub trait AdviceProvider {
    fn clone_merkle_set(&mut self, root: Word) -> Result<(), ExecutionError>;
}

In the implementation (e.g., MemAdviceProvider) this shouldn't be too difficult to support either - i.e., instead of using BTreeMap<[u8; 32], MerkleSet> we could use BTreeMap<[u8; 32], Vec<MerkleSet>> and then clone operation would add new item to the vector under the specified key.

• add a new method to the advice provider
• replace Assembly instruction mtree_cwm with mtree_clone
• update the docs

[grjte]

With the implementation of MerkleStore this will no longer be necessary