Slash and prove membership of prior sessions (#2970)

* skeleton for tracking historical sessions

* refactor OpaqueKeys

* some more skeleton work

* adjust session to new OpaqueKeys API

* further refactoring of key-type-ids

* session gets validator ID parameter

* run up against compiler

* tweak staking to support new session changes

* first run at child storage for deduplication

* Make session use `AccountId` as `ValidatorId`

* run up against child trie issues

* switch to using normal trie but with a fixed prefix

* clear out some println

* add dedup test

* flesh out historical module more

* introduce ExposureOf for staking

* test the historical module

* WASM compiles

* tests all compile

* do some mock change

* fix bulk of tests

* fix staking tests

* test obsolecence mechanic

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* some more style nits

* a couple more nits

* tweak tries

* fix typo thie -> this

substrate/srml/aura/src/mock.rs

@@ -18,7 +18,11 @@
 
 #![cfg(test)]
 
-use primitives::{traits::IdentityLookup, testing::{Header, UintAuthorityId}};
+use primitives::{
+	KeyTypeId,
+	traits::IdentityLookup,
+	testing::{UINT_DUMMY_KEY, Header, UintAuthorityId},
+};
 use srml_support::impl_outer_origin;
 use runtime_io;
 use substrate_primitives::{H256, Blake2Hasher};