Stop Importing Full Header Chain (#707)

* Make AncestryProof type more flexible

* Only import single finalized header instead of a chain

* Fix unchecked header import tests

* Add option for limiting ancestry proof size

* Update finality verifier Config in runtimes

* Update some documentation

* Fix Clippy warning

* Allow AncestryChecker to return proof size

Stops us from abusing the `Size` trait

* Remove Size impl for Vec<T>

* Remove size contraints for ancestry proofs

With different proof types its unclear how to "size" should be interpreted,
so we remove this requirement all together to avoid confusion.

bridges/modules/finality-verifier/Cargo.toml

@@ -11,7 +11,6 @@ license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
 codec = { package = "parity-scale-codec", version = "1.3.1", default-features = false }
 finality-grandpa = { version = "0.12.3", default-features = false }
 serde = { version = "1.0", optional = true }
-num-traits = { version = "0.2", default-features = false }
 
 # Bridge Dependencies
 
@@ -40,7 +39,6 @@ std = [
 	"finality-grandpa/std",
 	"frame-support/std",
 	"frame-system/std",
-	"num-traits/std",
 	"serde",
 	"sp-runtime/std",
 	"sp-std/std",