grandpa: Use storage proofs for Grandpa authorities (#3734)

* grandpa: Write Grandpa authorities to well known key.

Instead of requiring execution proofs for Grandpa authorities, this
enables much simpler storage proofs.

* grandpa: Introduce named AuthorityList type.

* grandpa: Storage migration for srml-grandpa module.

* Remove no-longer-used GrandpaApi runtime API.

* grandpa: Write AuthorityList to storage with encoding version.

We expect the AuthorityList type may change (eg. key changes). To make
upgrades smoother, include a version in the stored value.

* Bump node runtime spec version.

* Update srml/grandpa/src/lib.rs

Co-Authored-By: André Silva <andre.beat@gmail.com>

substrate/node-template/runtime/src/lib.rs

@@ -23,8 +23,7 @@ use client::{
 	runtime_api as client_api, impl_runtime_apis
 };
 use aura_primitives::sr25519::AuthorityId as AuraId;
-use grandpa::{AuthorityId as GrandpaId, AuthorityWeight as GrandpaWeight};
-use grandpa::fg_primitives;
+use grandpa::AuthorityId as GrandpaId;
 use version::RuntimeVersion;
 #[cfg(feature = "std")]
 use version::NativeVersion;
@@ -353,10 +352,4 @@ impl_runtime_apis! {
 			opaque::SessionKeys::generate(seed)
 		}
 	}
-
-	impl fg_primitives::GrandpaApi<Block> for Runtime {
-		fn grandpa_authorities() -> Vec<(GrandpaId, GrandpaWeight)> {
-			Grandpa::grandpa_authorities()
-		}
-	}
 }