Introduce a PhantomData field in GenesisConfig (#1116)

This makes it possible to support `GenesisConfig`s that to do not use
generic parameter.

substrate/srml/contract/src/tests.rs

@@ -131,6 +131,7 @@ impl ExtBuilder {
 				transfer_fee: self.transfer_fee,
 				creation_fee: self.creation_fee,
 				reclaim_rebate: 0,
+				_genesis_phantom_data: Default::default(),
 			}.build_storage()
 			.unwrap().0,
 		);
@@ -143,6 +144,7 @@ impl ExtBuilder {
 				max_depth: 100,
 				block_gas_limit: self.block_gas_limit,
 				current_schedule: Default::default(),
+				_genesis_phantom_data: Default::default(),
 			}.build_storage()
 			.unwrap().0,
 		);