Implement Substrate Pallet Runtime APIs (#389)

* Implement public helpers for querying header info

* Update `best_header` when importing headers

* Add BestHeader to GenesisConfig

* Define extra types for Millau primitives

* Start implementing runtime APIs in Millau runtime

* Add helper for getting headers which require a justification

* Add runtime API for getting headers requiring a justification

* Reword `expect()` proof for valid authority sets

* Fix typo

* Clean up Hasher comment

* Add the Call Dispatch Pallet back to the Millau runtime

* Use types from Rialto in bridge pallet config

* Use the Rialto runtime APIS in the Millau runtime

* Include Millau bridge instance in Rialto runtime

* Add missing doc comment

* Use one storage function for setting and clearing `RequiresJustification`

* Remove TODO comments