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/primitives/src/storage.rs

@@ -71,6 +71,9 @@ pub mod well_known_keys {
 	/// Current extrinsic index (u32) is stored under this key.
 	pub const EXTRINSIC_INDEX: &'static [u8] = b":extrinsic_index";
 
+	/// Sum of all lengths of executed extrinsics (u32).
+	pub const ALL_EXTRINSICS_LEN: &'static [u8] = b":all_extrinsics_len";
+
 	/// Changes trie configuration is stored under this key.
 	pub const CHANGES_TRIE_CONFIG: &'static [u8] = b":changes_trie";