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/node/executor/src/lib.rs

@@ -55,7 +55,8 @@ mod tests {
 	use node_runtime::{
 		Header, Block, UncheckedExtrinsic, CheckedExtrinsic, Call, Runtime, Balances, BuildStorage,
 		GenesisConfig, BalancesConfig, SessionConfig, StakingConfig, System, SystemConfig,
-		GrandpaConfig, IndicesConfig, ContractsConfig, Event, SessionKeys, CENTS, DOLLARS, MILLICENTS
+		GrandpaConfig, IndicesConfig, ContractsConfig, Event, SessionKeys,
+		CENTS, DOLLARS, MILLICENTS,
 	};
 	use wabt;
 	use primitives::map;
@@ -290,7 +291,9 @@ mod tests {
 	}
 
 	fn to_session_keys(ring: &AuthorityKeyring) -> SessionKeys {
-		SessionKeys(ring.to_owned().into(), ring.to_owned().into())
+		SessionKeys {
+			ed25519: ring.to_owned().into(),
+		}
 	}
 
 	fn new_test_ext(code: &[u8], support_changes_trie: bool) -> TestExternalities<Blake2Hasher> {