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/balances/src/lib.rs

@@ -1187,7 +1187,6 @@ pub struct TakeFees<T: Trait<I>, I: Instance = DefaultInstance>(#[codec(compact)
 
 impl<T: Trait<I>, I: Instance> TakeFees<T, I> {
 	/// utility constructor. Used only in client/factory code.
-	#[cfg(feature = "std")]
 	pub fn from(fee: T::Balance) -> Self {
 		Self(fee)
 	}