Expose some of session module's storage as public, as well as the set_keys transaction (#4175)

* Expose some of session's storage as public, as well as set_keys

Seemingly there's no reason not to do this, as anyone can always do
it the "hard way" by constructing storage keys or extrinsics.

* Use trait to expose `is_registered` function

* Missed removing a pub keyword

* Move trait to support, add docstrings

substrate/frame/support/src/traits.rs

@@ -769,3 +769,12 @@ pub trait Randomness<Output> {
 		Self::random(&[][..])
 	}
 }
+
+/// Implementors of this trait provide information about whether or not some validator has
+/// been registered with them. The [Session module](../../pallet_session/index.html) is an implementor.
+pub trait ValidatorRegistration<ValidatorId> {
+	/// Returns true if the provided validator ID has been registered with the implementing runtime
+	/// module
+	fn is_registered(id: &ValidatorId) -> bool;
+}
+