Fix #1536: do not require to construct a block for encoding it (#3813)

* Fix #1536: do not require to construct a block for encoding it

* Bump `impl_version`

* Improve `Block::encode_from` signature and rustdoc (from review by @bkchr)

substrate/core/sr-primitives/src/generic/block.rs

@@ -93,6 +93,9 @@ where
 	fn new(header: Self::Header, extrinsics: Vec<Self::Extrinsic>) -> Self {
 		Block { header, extrinsics }
 	}
+	fn encode_from(header: &Self::Header, extrinsics: &[Self::Extrinsic]) -> Vec<u8> {
+		(header, extrinsics).encode()
+	}
 }
 
 /// Abstraction over a substrate block and justification.

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

@@ -259,6 +259,9 @@ impl<Xt: 'static + Codec + Sized + Send + Sync + Serialize + Clone + Eq + Debug
 	fn new(header: Self::Header, extrinsics: Vec<Self::Extrinsic>) -> Self {
 		Block { header, extrinsics }
 	}
+	fn encode_from(header: &Self::Header, extrinsics: &[Self::Extrinsic]) -> Vec<u8> {
+		(header, extrinsics).encode()
+	}
 }
 
 impl<'a, Xt> Deserialize<'a> for Block<Xt> where Block<Xt>: Decode {

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

@@ -687,6 +687,8 @@ pub trait Block: Clone + Send + Sync + Codec + Eq + MaybeSerializeDebugButNotDes
 	fn hash(&self) -> Self::Hash {
 		<<Self::Header as Header>::Hashing as Hash>::hash_of(self.header())
 	}
+	/// Create an encoded block from the given `header` and `extrinsics` without requiring to create an instance.
+	fn encode_from(header: &Self::Header, extrinsics: &[Self::Extrinsic]) -> Vec<u8>;
 }
 
 /// Something that acts like an `Extrinsic`.