Ease parachain candidate code fetching (#2593)

* code stored in para + modify CandidateDescriptor.

* WIP: digest + some more impl

* validation_code_hash in payload + check in inclusion

* check in client + refator

* tests

* fix encoding indices

* remove old todos

* fix test

* fix test

* add test

* fetch validation code inside collation-generation from the relay-chain

* HashMismatch -> PoVHashMismatch + miscompilation

* refactor, store hash when needed

* storage rename: more specific but slightly too verbose

* do not hash on candidate validation, fetch hash instead

* better test

* fix test

* guide updates

* don't panic in runtime

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

polkadot/parachain/src/primitives.rs

@@ -21,6 +21,7 @@ use sp_std::vec::Vec;
 
 use parity_scale_codec::{Encode, Decode, CompactAs};
 use sp_core::{RuntimeDebug, TypeId};
+use sp_runtime::traits::Hash as _;
 
 #[cfg(feature = "std")]
 use serde::{Serialize, Deserialize};
@@ -45,7 +46,6 @@ pub struct HeadData(#[cfg_attr(feature = "std", serde(with="bytes"))] pub Vec<u8
 impl HeadData {
 	/// Returns the hash of this head data.
 	pub fn hash(&self) -> Hash {
-		use sp_runtime::traits::Hash;
 		sp_runtime::traits::BlakeTwo256::hash(&self.0)
 	}
 }
@@ -55,6 +55,13 @@ impl HeadData {
 #[cfg_attr(feature = "std", derive(Serialize, Deserialize, Hash, MallocSizeOf))]
 pub struct ValidationCode(#[cfg_attr(feature = "std", serde(with="bytes"))] pub Vec<u8>);
 
+impl ValidationCode {
+	/// Get the blake2-256 hash of the validation code bytes.
+	pub fn hash(&self) -> Hash {
+		sp_runtime::traits::BlakeTwo256::hash(&self.0[..])
+	}
+}
+
 /// Parachain block data.
 ///
 /// Contains everything required to validate para-block, may contain block and witness data.