Bring support for calling check_inherents (#490)

* Start

* More work

* Add proc-macro for `validate_block`

* Make everything compile

* Add some test

test/runtime/src/lib.rs

@@ -427,4 +427,29 @@ impl_runtime_apis! {
 	}
 }
 
-cumulus_pallet_parachain_system::register_validate_block!(Runtime, Executive);
+struct CheckInherents;
+
+impl cumulus_pallet_parachain_system::CheckInherents<Block> for CheckInherents {
+	fn check_inherents(
+		_: &[UncheckedExtrinsic],
+		relay_state_proof: &cumulus_pallet_parachain_system::RelayChainStateProof,
+	) -> sp_inherents::CheckInherentsResult {
+		if relay_state_proof.read_slot().expect("Reads slot") == 1337u64 {
+			let mut res = sp_inherents::CheckInherentsResult::new();
+			res.put_error(
+				[1u8; 8],
+				&sp_inherents::MakeFatalError::from("You are wrong"),
+			)
+			.expect("Puts error");
+			res
+		} else {
+			sp_inherents::CheckInherentsResult::new()
+		}
+	}
+}
+
+cumulus_pallet_parachain_system::register_validate_block! {
+	Runtime = Runtime,
+	BlockExecutor = Executive,
+	CheckInherents = CheckInherents,
+}