Move Externalities into its own crate (#3775)

* Move `Externalities` into `substrate-externalities`

- `Externalities` now support generic extensions
- Split of `primtives-storage` for storage primitive types

* Move the externalities scoping into `substrate-externalities`

* Fix compilation

* Review feedback

* Adds macro for declaring extensions

* Fix benchmarks

* Introduce `ExtensionStore` trait

* Last review comments

* Implement it for `ExtensionStore`

substrate/core/finality-grandpa/src/finality_proof.rs

@@ -48,7 +48,7 @@ use sr_primitives::{
 	Justification, generic::BlockId,
 	traits::{NumberFor, Block as BlockT, Header as HeaderT, One},
 };
-use primitives::{H256, Blake2Hasher, offchain::NeverOffchainExt};
+use primitives::{H256, Blake2Hasher};
 use substrate_telemetry::{telemetry, CONSENSUS_INFO};
 use fg_primitives::AuthorityId;
 
@@ -78,7 +78,7 @@ impl<B, E, Block: BlockT<Hash=H256>, RA> AuthoritySetForFinalityProver<Block> fo
 			"GrandpaApi_grandpa_authorities",
 			&[],
 			ExecutionStrategy::NativeElseWasm,
-			NeverOffchainExt::new(),
+			None,
 		).and_then(|call_result| Decode::decode(&mut &call_result[..])
 			.map_err(|err| ClientError::CallResultDecode(
 				"failed to decode GRANDPA authorities set proof".into(), err