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/application-crypto/src/ed25519.rs

@@ -23,6 +23,10 @@ pub use primitives::ed25519::*;
 mod app {
 	use primitives::testing::ED25519;
 	crate::app_crypto!(super, ED25519);
+
+	impl crate::traits::BoundToRuntimeAppPublic for Public {
+		type Public = Self;
+	}
 }
 
 pub use app::Public as AppPublic;

substrate/core/application-crypto/src/sr25519.rs

@@ -23,6 +23,10 @@ pub use primitives::sr25519::*;
 mod app {
 	use primitives::testing::SR25519;
 	crate::app_crypto!(super, SR25519);
+
+	impl crate::traits::BoundToRuntimeAppPublic for Public {
+		type Public = Self;
+	}
 }
 
 pub use app::Public as AppPublic;

substrate/core/application-crypto/src/traits.rs

@@ -127,3 +127,8 @@ pub trait RuntimeAppPublic: Sized  {
 	fn verify<M: AsRef<[u8]>>(&self, msg: &M, signature: &Self::Signature) -> bool;
 }
 
+/// Something that bound to a fixed `RuntimeAppPublic`.
+pub trait BoundToRuntimeAppPublic {
+	/// The `RuntimeAppPublic` this type is bound to.
+	type Public: RuntimeAppPublic;
+}