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/babe/src/lib.rs

@@ -218,6 +218,7 @@ impl<T: Trait> OnTimestampSet<T::Moment> for Module<T> {
 
 impl<T: Trait> session::OneSessionHandler<T::AccountId> for Module<T> {
 	type Key = AuthorityId;
+
 	fn on_new_session<'a, I: 'a>(changed: bool, validators: I)
 		where I: Iterator<Item=(&'a T::AccountId, AuthorityId)>
 	{