title: Computed is not assignable to Observable after upgrade to Knockout.js 3.5.0
body: <p>Knockout.js 3.5.0 comes with it's own type definitions on npm.
  Let's have this variable <code>x</code> defined as:</p>
  
  <pre><code>const x: ko.Observable&lt;boolean&gt; = ko.computed(() =&gt; true);
  </code></pre>
  
  <p>Now we got compiler error: 
  <em>Type 'Computed' is missing the following properties from type 'Observable': valueHasMutated, valueWillMutate</em>.</p>
  
  <p>I understand what compiler is trying to tell me, but I think that principally this is wrong - because every computed is also an observable. This behaviour worked well on 3.4.x using type definitions from @types/knockout.</p>
  
  <p>There is the <code>Subscribable</code> type, which I believe is some "parent" type, and it works in declaration:</p>
  
  <pre><code>const x: ko.Subscribable&lt;boolean&gt; = ko.computed(() =&gt; true);
  </code></pre>
  
  <p>This throws no error. However no type checking is performed on value setter:</p>
  
  <pre><code>x(2);    // this is wrong but no compiler error
  </code></pre>
  
  <p>Can anybody shed some light on these new type hierarchy changes and how should we use it?</p>