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/relays/finality/src/lib.rs

@@ -21,6 +21,7 @@
 
 pub use crate::finality_loop::{run, FinalitySyncParams, SourceClient, TargetClient};
 
+use bp_header_chain::FinalityProof;
 use std::fmt::Debug;
 
 mod finality_loop;
@@ -50,9 +51,3 @@ pub trait SourceHeader<Number>: Clone + Debug + PartialEq + Send + Sync {
 	/// Returns true if this header needs to be submitted to target node.
 	fn is_mandatory(&self) -> bool;
 }
-
-/// 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;
-}