Allow both consensus and runtime to limit block building (#1581)

* Limit block size in runtime,

* Add test for basic authorship.

* Store length of extrinsics instead of computing it.

* Don't rely on note_extrinsic

* Use hashed version of storage and write test.

* Recompile runtime.

substrate/core/client/src/block_builder/block_builder.rs

@@ -76,9 +76,9 @@ where
 		})
 	}
 
-	/// Push onto the block's list of extrinsics. This will ensure the extrinsic
-	/// can be validly executed (by executing it); if it is invalid, it'll be returned along with
-	/// the error. Otherwise, it will return a mutable reference to self (in order to chain).
+	/// Push onto the block's list of extrinsics.
+	///
+	/// This will ensure the extrinsic can be validly executed (by executing it);
 	pub fn push(&mut self, xt: <Block as BlockT>::Extrinsic) -> error::Result<()> {
 		use crate::runtime_api::ApiExt;