title: RxSwift - withLatestFrom combining values from both observables
body: <p>I want to achieve result like this:</p>
  
  <pre><code>L -1-2-3------4------5-6-7-8----
  R ---------A------B----------C--
  
  O ---------A3-----B4---------C8
  </code></pre>
  
  <p>So basically something like withLatestFrom but combining values from both observables (like combine latest).</p>
  
  <p>I guess there is no ready operator for that. Any idea how to achieve this?</p>