Implement hArk round protocol

This PR is pretty big, I tried to chop it up into pieces at least a little bit.

There are three major parts:

• the lib commit that refactors the ark::tree::signed module to be hArk-based
  • comes with some changes required to server and client to make the compiler happy
• a bunch of preparatory commits in the server to add hArk functionality without changing the protocol yet
• the actual protocol implementation
  • server and client in separate commits

Currently missing:

• reimplementation of the sweeper and forfeit watcher (they are neutralized in this PR)
• bark API to actually participate in non-interactive rounds
  • accompanying integration tests for this functionality

This is a breaking change without any effort made to stay backwards compatible.

Follow-up issues:

• sweeper/watcher refactor: &3
• exit failed forfeit exchange: #1326