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.