Node-side subsystems for Disputes (#2566)

* dispute subsystem files

* rename

* fix linkcheck

* flesh out section README

* coordinator schema

* DisputeCoordinatorMessage

* stub & coordinator protocol

* dispute coordinator

* add some more message fields

* move links to bottom

* dispute participation

* Cleen It Up !

* runtime: store candidate receipts in dispute state

yeah, this is a little heavier. why are you reading this?

* Revert "runtime: store candidate receipts in dispute state"

This reverts commit 51c10bfd4d866e287e6bd88f317ed57ed987eaee.

* add dispute availability statement type and prepare for availability

* add 'spam slots' to disputes runtmie

* return Spam Slots info from runtime

* rework `ImportStatement` to `ImportStatements`

* some more methods for dispute coordinator

* candidates-included runtime API

* algo for providing disputes to runtime.

* handle signing with coordinator

* dispute coordinator chain ops

* remove dead file

* remove keystore from dispute participation

* adjust ApprovedAncestor to return the necssary data

* discuss how approved ancestor and determine undisputed chain are used together

* add TODO

* initiate disputes from approval voting

* route statements from candidate backing and approval voting

* fix guide build

polkadot/roadmap/implementers-guide/src/runtime-api/candidates-included.md

@@ -0,0 +1,8 @@
+# Candidates Included
+
+This runtime API is for checking which candidates have been included within the chain, locally.
+
+```rust
+/// Input and output have the same length.
+fn candidates_included(Vec<(SessionIndex, CandidateHash)>) -> Vec<bool>;
+```