Weight annotation for block hooks. (#4058)

* Initial version that works with proper tests.

* get rid of todos and grumbles and such.

* Cleanup and fix line-width

* fix test runtime test

substrate/core/sr-primitives/src/testing.rs

@@ -145,7 +145,7 @@ pub type DigestItem = generic::DigestItem<H256>;
 pub type Digest = generic::Digest<H256>;
 
 /// Block Header
-#[derive(PartialEq, Eq, Clone, Serialize, Debug, Encode, Decode)]
+#[derive(PartialEq, Eq, Clone, Serialize, Debug, Encode, Decode, Default)]
 #[serde(rename_all = "camelCase")]
 #[serde(deny_unknown_fields)]
 pub struct Header {
@@ -198,6 +198,16 @@ impl traits::Header for Header {
 	}
 }
 
+impl Header {
+	/// A new header with the given number and default hash for all other fields.
+	pub fn new_from_number(number: <Self as traits::Header>::Number) -> Self {
+		Self {
+			number,
+			..Default::default()
+		}
+	}
+}
+
 impl<'a> Deserialize<'a> for Header {
 	fn deserialize<D: Deserializer<'a>>(de: D) -> Result<Self, D::Error> {
 		let r = <Vec<u8>>::deserialize(de)?;