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/test-runtime/src/lib.rs

@@ -226,7 +226,7 @@ impl_runtime_apis! {
 		}
 
 		fn inherent_extrinsics(_data: InherentData) -> Vec<<Block as BlockT>::Extrinsic> {
-			unimplemented!()
+			vec![]
 		}
 
 		fn check_inherents(_block: Block, _data: InherentData) -> CheckInherentsResult {