Additional derive cleanups (#4307)

* Additional derive cleanups

* Adjust

* Multi cache set

* Adjust mapping

packages/api-derive/src/util/index.ts

@@ -1,8 +1,16 @@
 // Copyright 2017-2021 @polkadot/api-derive authors & contributors
 // SPDX-License-Identifier: Apache-2.0
 
+import type { Observable } from 'rxjs';
+
+import { map } from 'rxjs';
+
 export { drr, memo } from '@polkadot/rpc-core';
 
 export * from './approvalFlagsToBools';
 export * from './cache';
 export * from './cacheImpl';
+
+export function firstObservable <T> (obs: Observable<T[]>): Observable<T> {
+  return obs.pipe(map(([a]) => a));
+}