Add a detector for singly-hashed Merkle leaves using OZ's MerkleProof library

### Describe the desired feature

https://www.rareskills.io/post/merkle-tree-second-preimage-attack

skip to the image with the green underlines. We should have a detector that can identify if the `leaf` variable has had only 1 keccack used to generate it, or none at all.

The case where only 1 keccak has been used should be a very good heuristic. The case where no keccaks have been used would be good but likely to have more false positives.