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/node/runtime/src/lib.rs

@@ -85,7 +85,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 175,
-	impl_version: 175,
+	impl_version: 176,
 	apis: RUNTIME_API_VERSIONS,
 };