"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/Cargo.toml

@@ -28,6 +28,7 @@ impl-trait-for-tuples = "0.1.3"
 sp-inherents = { version = "2.0.0-rc3", default-features = false, path = "../inherents" }
 parity-util-mem = { version = "0.6.1", default-features = false, features = ["primitive-types"] }
 hash256-std-hasher = { version = "0.15.2", default-features = false }
+either = { version = "1.5", default-features = false }
 
 [dev-dependencies]
 serde_json = "1.0.41"
@@ -51,4 +52,5 @@ std = [
 	"sp-inherents/std",
 	"parity-util-mem/std",
 	"hash256-std-hasher/std",
+	"either/use_std",
 ]

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