dirauth consensus algorithm

Arti dirauth will need to be able to calculate consensuses given votes. (Votes can come from C Tor or from Arti dirauth #1928, and are exchanged via dircache #1924.)

• New netdoc parser/printer (try to make a new derive macro) !3135 (merged)
• Encode whole of vote/consensus doc in format for new derive macro
  • Use multiple-include technique to to prepare for defining votes as well as md consensuses, removing the generic on NetworkStatus.
  • Derive (or implement) parse2 traits for types in the main part of tor-netdoc.
  • Implement a derive macro for netdoc writing - this can be done with the existing traits, I think.
    • Ensure that the new writing algorithm sorts its output (for reproducibility)
  • Implement the printing traits for votes (by derive)
• Function for calculating consensus field-by-field
• CLI utility for testing the votes-to-consensus calculator
• Plugin for C Tor
• Changes to C Tor to call the plugin (needs help from a C Tor dev)