Use GrandpaJustification instead of Vec<u8> in Pallet API (#847)

* Stop passing raw encoded justifications to pallet API

By having the API accept a struct-ified justification we are able to
better utilize the justifications fields for weight calculations.

* Update relayer code to use decoded justifications

* Add justification to `expect()` statement

* Fix some imports

* Make justification wrapper contain decoded justification

* Rename some fields

* Get rid of warnings

* Appease Clippy

* Only decode justification once at init time

* Remove unnecessary method

* Remove justification wrapper

This became kinda unnecessary since we could implement the FinalityProof
trait on GrandpaJustification directly.

bridges/primitives/header-chain/src/lib.rs

@@ -94,6 +94,12 @@ impl<H: Default, E> HeaderChain<H, E> for () {
 	}
 }
 
+/// Abstract finality proof that is justifying block finality.
+pub trait FinalityProof<Number>: Clone + Send + Sync + Debug {
+	/// Return number of header that this proof is generated for.
+	fn target_header_number(&self) -> Number;
+}
+
 /// Find header digest that schedules next GRANDPA authorities set.
 pub fn find_grandpa_authorities_scheduled_change<H: HeaderT>(
 	header: &H,