Incremental Merkle Tree

[stechu]

As we are building manta (https://github.com/Manta-Network), we figured out that we need an incremental merkle tree that has Log N internal nodes, which is not supported in the current MerkleTree implementaion. The properties that we need for this merkle tree:

1. Able to generate ZK membership proofs
2. The merkle tree can be fixed sized and append only (i.e. mark the unused leaf node as a filler, etc)
3. The number of internal nodes is Log N. (since we need to store the internal nodes on chain, O(N) storage would be too expensive).
   I think 3 is only possible if this merkle tree only support append. Happy to submit a PR on this. Would also like to hear is there any design considerations that need to take care of during the implementation.

[burdges]

https://github.com/arkworks-rs/sponge/