"OR gate" for EnsureOrigin (#6237)

* 'OR gate' for EnsureOrigin.

* Formatting.

* More formatting.

* Add docstring; Update 'Success' type.

* Bump runtime impl_version.

* Fix successful_origin.

* Add either into std feature list.

* Update docs.

substrate/primitives/runtime/src/lib.rs

@@ -81,6 +81,8 @@ pub use sp_arithmetic::biguint;
 
 pub use random_number_generator::RandomNumberGenerator;
 
+pub use either::Either;
+
 /// An abstraction over justification for a block's validity under a consensus algorithm.
 ///
 /// Essentially a finality proof. The exact formulation will vary between consensus