Per-things trait. (#4904)

* Give perthigns the trait it always deserved.

* Make staking and phragmen work with the new generic per_thing

* Make everything work together 🔨

* a bit of cleanup

* Clean usage

* Bump.

* Fix name

* fix grumbles

* hopefully fix the ui test

* Some grumbles

* revamp traits again

* Better naming again.

substrate/frame/session/src/mock.rs

@@ -21,8 +21,9 @@ use std::cell::RefCell;
 use frame_support::{impl_outer_origin, parameter_types, weights::Weight};
 use sp_core::{crypto::key_types::DUMMY, H256};
 use sp_runtime::{
-	Perbill, impl_opaque_keys, traits::{BlakeTwo256, IdentityLookup, ConvertInto},
-	testing::{Header, UintAuthorityId}
+	Perbill, impl_opaque_keys,
+	traits::{BlakeTwo256, IdentityLookup, ConvertInto},
+	testing::{Header, UintAuthorityId},
 };
 use sp_staking::SessionIndex;