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

frame/session/src/lib.rs

@@ -126,7 +126,7 @@ use support::weights::SimpleDispatchInfo;
 use sp_runtime::traits::{Convert, Zero, Member, OpaqueKeys};
 use sp_staking::SessionIndex;
 use support::{dispatch::Result, ConsensusEngineId, decl_module, decl_event, decl_storage};
-use support::{ensure, traits::{OnFreeBalanceZero, Get, FindAuthor}, Parameter};
+use support::{ensure, traits::{OnFreeBalanceZero, Get, FindAuthor, ValidatorRegistration}, Parameter};
 use system::{self, ensure_signed};
 
 #[cfg(test)]
@@ -333,6 +333,12 @@ impl<V> SelectInitialValidators<V> for () {
 	}
 }
 
+impl<T: Trait> ValidatorRegistration<T::ValidatorId> for Module<T> {
+	fn is_registered(id: &T::ValidatorId) -> bool {
+		Self::load_keys(id).is_some()
+	}
+}
+
 pub trait Trait: system::Trait {
 	/// The overarching event type.
 	type Event: From<Event> + Into<<Self as system::Trait>::Event>;

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;
+}
+