Implement XMSS library

[sea212]

To unite multiple public keys into one hash (public key) and additionally allow for O(log n) verification that a public key is part of that composition, a merkle trie is required that contains all the public keys.

ToDo:

• Create XMSS class
• Implement merkle trie that contains a set of public keys
• Add functionality for XMSS tree generation using WOTS keys from this package - also creates a list
• Documentation and testing