Add more to spec for generalising structure and features of trees and mixers.

[drewstone]

The running notion document contains information about what features we would like to generalise over. We should continue to add to this and help flush out a more balanced implementation that feels production ready.

Additionally, there's a variety of tree types, commitment types, and element types we want to support for future use cases. We should aim to build a better API over this. For example, we have Curve25519 working directly over this now, but with more research into Arkworks we may want to support the PrimeField type. How we should we go about merging these should be answered in order to complete this issue.

[drewstone]

https://www.notion.so/hicommonwealth/Merkle-Functionality-96e157c7190b445d9a981d6124f6aaac