title: NSString to UInt64
body: <p>How do you convert from <code>NSString</code> to <code>UInt64</code>?</p>
  
  <p>For example, something like this if a <code>UInt64Value</code> helper method existed:</p>
  
  <pre><code>NSString *value = @"1234567";
  UInt64 convertedValue = [value UInt64Value];
  </code></pre>
  
  <p>I am trying to do this in an iOS project.</p>