Parachains double vote handler initial implementation. (#840)

* Parachains double vote handler initial implementation.

* Make tests test the actual slashing.

* Implement SignedExtension validation of double vote reports.

* Fixes build after merge

* Review fixes

* Adds historical session proofs

* Review fixes.

* Bump runtime spec_version

* Get the session number from the proof

* Check that proof matches session

* Change signature type on DoubleVoteReport

* Adds docs and removes blank lines

* Removes leftover code

* Fix build

* Fix build after a merge

* Apply suggestions from code review

Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

* Prune ParentToSessionIndex

* Remove a clone and a warning

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>
Co-authored-by: Gavin Wood <gavin@parity.io>
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