Remove footgun around session keys/handlers (#3949)

* Remove footgun around session keys/handlers

- `OpaqueKeys` now has an associated type `KeyTypeIdProviders`. This can
be used in the runtime as input for `SessionHandler` from the session
trait.
- `impl_opaque_keys` now works with modules and extracts the `KeyTypeId`
from the module directly.
- Added some checks to the `session` storage initialization that checks
that the `SessionHandler` and `Keys` use the same number of keys and
that the order is equal.

* Update core/sr-primitives/src/traits.rs

substrate/srml/aura/src/lib.rs

@@ -182,6 +182,10 @@ impl<T: Trait> Module<T> {
 	}
 }
 
+impl<T: Trait> sr_primitives::BoundToRuntimeAppPublic for Module<T> {
+	type Public = T::AuthorityId;
+}
+
 impl<T: Trait> session::OneSessionHandler<T::AccountId> for Module<T> {
 	type Key = T::AuthorityId;