Add mwixnet code to master

[yeastplume]

WIP, moving mwixnet comsig creation code + libraries into master branch (were previously only in contracts)

• [x] Introduce comsig functions + code and update crates
• [x] Clean up of redundant secp256k1 calls
• [x] Introduction + Documentation of comsig API calls
• [x] Reimplementation of unit tests