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/Cargo.toml

@@ -10,6 +10,7 @@ description = "Finality proofs relay"
 async-std = "1.6.5"
 async-trait = "0.1.40"
 backoff = "0.2"
+bp-header-chain = { path = "../../primitives/header-chain" }
 futures = "0.3.5"
 headers-relay = { path = "../headers" }
 log = "0.4.11"

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;
-}