* set up data types and control flow for statement distribution
* add some set-like methods to View
* implement sending to peers
* start fixing equivocation handling
* Add a section to the statement distribution subsystem on equivocations and flood protection
* fix typo and amend wording
* implement flood protection
* have peer knowledge tracker follow when peer first learns about a candidate
* send dependents after circulating
* add another TODO
* trigger send in one more place
* refactors from review
* send new statements to candidate backing
* instantiate active head data with runtime API values
* track our view changes and peer view changes
* apply a benefit to peers who send us statements we want
* remove unneeded TODO
* add some comments and improve Hash implementation
* start tests and fix `note_statement`
* test active_head seconding logic
* test that the per-peer tracking logic works
* test per-peer knowledge tracker
* test that peer view updates lead to messages being sent
* test statement circulation
* address review comments
* have view set methods return references
Fedor Sakharov