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/core/sr-primitives/src/lib.rs

@@ -51,6 +51,9 @@ pub mod transaction_validity;
 /// Re-export these since they're only "kind of" generic.
 pub use generic::{DigestItem, Digest};
 
+/// Re-export this since it's part of the API of this crate.
+pub use substrate_primitives::crypto::{key_types, KeyTypeId};
+
 /// A message indicating an invalid signature in extrinsic.
 pub const BAD_SIGNATURE: &str = "bad signature in extrinsic";