Introduce a PhantomData field in GenesisConfig (#1116)

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

substrate/srml/session/src/lib.rs

@@ -270,13 +270,16 @@ mod tests {
 		t.extend(consensus::GenesisConfig::<Test>{
 			code: vec![],
 			authorities: vec![1, 2, 3],
+			_genesis_phantom_data: Default::default(),
 		}.build_storage().unwrap().0);
 		t.extend(timestamp::GenesisConfig::<Test>{
 			period: 5,
+			_genesis_phantom_data: Default::default(),
 		}.build_storage().unwrap().0);
 		t.extend(GenesisConfig::<Test>{
 			session_length: 2,
 			validators: vec![1, 2, 3],
+			_genesis_phantom_data: Default::default(),
 		}.build_storage().unwrap().0);
 		runtime_io::TestExternalities::new(t)
 	}