add AggregationTree

[enricobottazzi]

#172

• Create a new AggregationMerkleSumTree data structure to merkle_sum_tree module
• Add test to generate MstInclusionProof starting from AggregationMerkleSumTree

[alxkzmn]

It may be a good time to refactor the MST-related functions inside the utils and turn them into the MST trait. Now, after you have introduced an aggregate tree, some of the functions serve both data structures (the old "mini" tree and the aggregate tree). It would be nice to have them both implementing an "MST" trait that has the default implementation for shared methods like getting the Merkle proof. This way I could simply write some_mst.get_proof(&leaf) - that's what I expect when working with a language that allows for OOP. Having data structures and functions completely separate is not as intuitive - if I'm a new developer coming to the project, I would have to search for the Merkle proof function to find out that it actually lives in a separate file in a separate module.
See here, for example: https://doc.rust-lang.org/book/ch10-02-traits.html#using-trait-bounds-to-conditionally-implement-methods

[enricobottazzi]

@alxkzmn great point! gonna apply the changes