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/staking/src/mock.rs

@@ -17,12 +17,12 @@
 //! Test utilities
 
 use std::{collections::HashSet, cell::RefCell};
-use sr_primitives::Perbill;
+use sr_primitives::{Perbill, KeyTypeId};
 use sr_primitives::curve::PiecewiseLinear;
 use sr_primitives::traits::{IdentityLookup, Convert, OpaqueKeys, OnInitialize, SaturatedConversion};
 use sr_primitives::testing::{Header, UintAuthorityId};
 use sr_staking_primitives::SessionIndex;
-use primitives::H256;
+use primitives::{H256, crypto::key_types};
 use runtime_io;
 use support::{assert_ok, impl_outer_origin, parameter_types, StorageLinkedMap};
 use support::traits::{Currency, Get, FindAuthor};
@@ -52,6 +52,8 @@ thread_local! {
 
 pub struct TestSessionHandler;
 impl session::SessionHandler<AccountId> for TestSessionHandler {
+	const KEY_TYPE_IDS: &'static [KeyTypeId] = &[key_types::DUMMY];
+
 	fn on_genesis_session<Ks: OpaqueKeys>(_validators: &[(AccountId, Ks)]) {}
 
 	fn on_new_session<Ks: OpaqueKeys>(