Submit (& sign) extrinsics from the runtime. (#3514)

* Abstract constructing extrinsic and signing.

* Initial impl of signer.

* Implement get payload.

* Clean up the code.

* Improve docs.

* Bump version.

* Update core/sr-primitives/src/generic/unchecked_extrinsic.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix tests & address grumbles.

* Fix build.

* Fix runtime tests.

* Fix bound test.

* Fix bound test.

substrate/srml/support/src/inherent.rs

@@ -55,14 +55,16 @@ macro_rules! impl_outer_inherent {
 			fn create_extrinsics(&self) ->
 				$crate::inherent::Vec<<$block as $crate::inherent::BlockT>::Extrinsic> {
 				use $crate::inherent::ProvideInherent;
+				use $crate::inherent::Extrinsic;
 
 				let mut inherents = Vec::new();
 
 				$(
 					if let Some(inherent) = $module::create_inherent(self) {
-						inherents.push($uncheckedextrinsic::new_unsigned(
-							Call::$call(inherent))
-						);
+						inherents.push($uncheckedextrinsic::new(
+							Call::$call(inherent),
+							None,
+						).expect("Runtime UncheckedExtrinsic is not Opaque, so it has to return `Some`; qed"));
 					}
 				)*