message: refactor(interfaces): make UnaryFunction and OperatorFunction more concise
  Thanks, David!
diff: import { Observable } from './Observable';
  -export interface UnaryFunction<T, R> {
  - (source: T): R;
  -}
  +export type UnaryFunction<T, R> = (source: T) => R;
  -export interface OperatorFunction<T, R> extends UnaryFunction<Observable<T>, Observable<R>> {
  -}
  +export type OperatorFunction<T, R> = UnaryFunction<Observable<T>, Observable<R>>;
  