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/runtime/src/lib.rs

@@ -29,7 +29,6 @@ use node_primitives::{
 	AccountId, AccountIndex, Balance, BlockNumber, Hash, Index,
 	Moment, Signature,
 };
-use grandpa::fg_primitives;
 use client::{
 	block_builder::api::{self as block_builder_api, InherentData, CheckInherentsResult},
 	runtime_api as client_api, impl_runtime_apis
@@ -46,7 +45,7 @@ use version::RuntimeVersion;
 #[cfg(any(feature = "std", test))]
 use version::NativeVersion;
 use primitives::OpaqueMetadata;
-use grandpa::{AuthorityId as GrandpaId, AuthorityWeight as GrandpaWeight};
+use grandpa::AuthorityId as GrandpaId;
 use im_online::sr25519::{AuthorityId as ImOnlineId};
 use transaction_payment_rpc_runtime_api::RuntimeDispatchInfo;
 use contracts_rpc_runtime_api::ContractExecResult;
@@ -81,7 +80,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to equal spec_version. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 190,
+	spec_version: 191,
 	impl_version: 191,
 	apis: RUNTIME_API_VERSIONS,
 };
@@ -611,12 +610,6 @@ impl_runtime_apis! {
 		}
 	}
 
-	impl fg_primitives::GrandpaApi<Block> for Runtime {
-		fn grandpa_authorities() -> Vec<(GrandpaId, GrandpaWeight)> {
-			Grandpa::grandpa_authorities()
-		}
-	}
-
 	impl babe_primitives::BabeApi<Block> for Runtime {
 		fn configuration() -> babe_primitives::BabeConfiguration {
 			// The choice of `c` parameter (where `1 - c` represents the