Make CandidateHash a real type (#1916)

* Make `CandidateHash` a real type

This pr adds a new type `CandidateHash` that is used instead of the
opaque `Hash` type. This helps to ensure on the type system level that
we are passing the correct types.

This pr also fixes wrong usage of `relay_parent` as `candidate_hash`
when communicating with the av storage.

* Update core-primitives/src/lib.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Wrap the lines

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

polkadot/node/primitives/src/lib.rs

@@ -28,7 +28,7 @@ use polkadot_primitives::v1::{
 	Hash, CommittedCandidateReceipt, CandidateReceipt, CompactStatement,
 	EncodeAs, Signed, SigningContext, ValidatorIndex, ValidatorId,
 	UpwardMessage, ValidationCode, PersistedValidationData, ValidationData,
-	HeadData, PoV, CollatorPair, Id as ParaId, ValidationOutputs,
+	HeadData, PoV, CollatorPair, Id as ParaId, ValidationOutputs, CandidateHash,
 };
 use polkadot_statement_table::{
 	generic::{
@@ -54,10 +54,10 @@ pub enum Statement {
 	Seconded(CommittedCandidateReceipt),
 	/// A statement that a validator has deemed a candidate valid.
 	#[codec(index = "2")]
-	Valid(Hash),
+	Valid(CandidateHash),
 	/// A statement that a validator has deemed a candidate invalid.
 	#[codec(index = "3")]
-	Invalid(Hash),
+	Invalid(CandidateHash),
 }
 
 impl Statement {