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/core/sr-primitives/src/testing.rs

@@ -117,10 +117,10 @@ impl app_crypto::RuntimeAppPublic for UintAuthorityId {
 }
 
 impl OpaqueKeys for UintAuthorityId {
-	type KeyTypeIds = std::iter::Cloned<std::slice::Iter<'static, KeyTypeId>>;
+	type KeyTypeIdProviders = ();
 
-	fn key_ids() -> Self::KeyTypeIds {
-		[key_types::DUMMY].iter().cloned()
+	fn key_ids() -> &'static [KeyTypeId] {
+		&[key_types::DUMMY]
 	}
 
 	fn get_raw(&self, _: KeyTypeId) -> &[u8] {
@@ -132,6 +132,10 @@ impl OpaqueKeys for UintAuthorityId {
 	}
 }
 
+impl crate::BoundToRuntimeAppPublic for UintAuthorityId {
+	type Public = Self;
+}
+
 /// Digest item
 pub type DigestItem = generic::DigestItem<H256>;