Move create_inherents into the block-builder (#6553)

* Move `create_inherents` into the block-builder

This moves the `create_inherents` call into the block-builder. This has
the advantage that `create_inherents` will be able to reuse the same
context that will be used when applying the extrinsics and we also save
one call to `on_initialize`. To make sure that `create_inherents` does
not modify any state, we execute it in a transaction that is
rolled-back after doing the runtime call.

* Feedback and build fix

* Update primitives/runtime/src/lib.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* Update client/block-builder/src/lib.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

substrate/primitives/runtime/src/lib.rs

@@ -801,6 +801,23 @@ impl Drop for SignatureBatching {
 	}
 }
 
+/// Describes on what should happen with a storage transaction.
+pub enum TransactionOutcome<R> {
+	/// Commit the transaction.
+	Commit(R),
+	/// Rollback the transaction.
+	Rollback(R),
+}
+
+impl<R> TransactionOutcome<R> {
+	/// Convert into the inner type.
+	pub fn into_inner(self) -> R {
+		match self {
+			Self::Commit(r) => r,
+			Self::Rollback(r) => r,
+		}
+	}
+}
 
 #[cfg(test)]
 mod tests {