create newtype for ValidationCodeHash (#3212)

* create newtype for ValidationCodeHash

* pvf: fix tests compilation

* primitives: fix test compilation

polkadot/roadmap/implementers-guide/src/runtime-api/validation-code.md

@@ -9,5 +9,5 @@ fn validation_code(at: Block, ParaId, OccupiedCoreAssumption) -> Option<Validati
 Fetch the validation code (past, present or future) by its hash.
 
 ```rust
-fn validation_code_by_hash(at: Block, Hash) -> Option<ValidationCode>;
+fn validation_code_by_hash(at: Block, ValidationCodeHash) -> Option<ValidationCode>;
 ```

polkadot/roadmap/implementers-guide/src/runtime/paras.md

@@ -117,9 +117,9 @@ ParaLifecycle: map ParaId => Option<ParaLifecycle>,
 /// The head-data of every registered para.
 Heads: map ParaId => Option<HeadData>;
 /// The validation code hash of every live para.
-CurrentCodeHash: map ParaId => Option<Hash>;
+CurrentCodeHash: map ParaId => Option<ValidationCodeHash>;
 /// Actual past code hash, indicated by the para id as well as the block number at which it became outdated.
-PastCodeHash: map (ParaId, BlockNumber) => Option<Hash>;
+PastCodeHash: map (ParaId, BlockNumber) => Option<ValidationCodeHash>;
 /// Past code of parachains. The parachains themselves may not be registered anymore,
 /// but we also keep their code on-chain for the same amount of time as outdated code
 /// to keep it available for secondary checkers.
@@ -136,15 +136,15 @@ PastCodePruning: Vec<(ParaId, BlockNumber)>;
 /// in the context of a relay chain block with a number >= `expected_at`.
 FutureCodeUpgrades: map ParaId => Option<BlockNumber>;
 /// The actual future code of a para.
-FutureCodeHash: map ParaId => Option<Hash>;
+FutureCodeHash: map ParaId => Option<ValidationCodeHash>;
 /// The actions to perform during the start of a specific session index.
 ActionsQueue: map SessionIndex => Vec<ParaId>;
 /// Upcoming paras instantiation arguments.
 UpcomingParasGenesis: map ParaId => Option<ParaGenesisArgs>;
 /// The number of references on the validation code in `CodeByHash` storage.
-CodeByHashRefs: map Hash => u32;
+CodeByHashRefs: map ValidationCodeHash => u32;
 /// Validation code stored by its hash.
-CodeByHash: map Hash => Option<ValidationCode>
+CodeByHash: map ValidationCodeHash => Option<ValidationCode>
 ```
 
 ## Session Change

polkadot/roadmap/implementers-guide/src/types/candidate.md

@@ -87,6 +87,8 @@ struct CandidateDescriptor {
 	signature: CollatorSignature,
 	/// Hash of the para header that is being generated by this candidate.
 	para_head: Hash,
+	/// The blake2-256 hash of the validation code bytes.
+	validation_code_hash: ValidationCodeHash,
 }
 ```