Parachains double vote handler initial implementation. (#840)

* Parachains double vote handler initial implementation.

* Make tests test the actual slashing.

* Implement SignedExtension validation of double vote reports.

* Fixes build after merge

* Review fixes

* Adds historical session proofs

* Review fixes.

* Bump runtime spec_version

* Get the session number from the proof

* Check that proof matches session

* Change signature type on DoubleVoteReport

* Adds docs and removes blank lines

* Removes leftover code

* Fix build

* Fix build after a merge

* Apply suggestions from code review

Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

* Prune ParentToSessionIndex

* Remove a clone and a warning

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>
Co-authored-by: Gavin Wood <gavin@parity.io>

polkadot/primitives/src/parachain.rs

@@ -580,7 +580,7 @@ pub struct Activity(#[cfg_attr(feature = "std", serde(with="bytes"))] pub Vec<u8
 
 /// Statements that can be made about parachain candidates. These are the
 /// actual values that are signed.
-#[derive(Clone, PartialEq, Eq, Encode)]
+#[derive(Clone, PartialEq, Eq, Encode, Decode)]
 #[cfg_attr(feature = "std", derive(Debug))]
 pub enum Statement {
 	/// Proposal of a parachain candidate.
@@ -596,8 +596,7 @@ pub enum Statement {
 
 /// An either implicit or explicit attestation to the validity of a parachain
 /// candidate.
-#[derive(Clone, PartialEq, Decode, Encode)]
-#[cfg_attr(feature = "std", derive(Debug))]
+#[derive(Clone, Eq, PartialEq, Decode, Encode, RuntimeDebug)]
 pub enum ValidityAttestation {
 	/// Implicit validity attestation by issuing.
 	/// This corresponds to issuance of a `Candidate` statement.