title: How to get subslices?
body: <p>I have a variable <code>a</code> of type <code>&amp;[T]</code>; how can I get a reference to a subslice of <code>a</code>? </p>
  
  <p>As a concrete example, I'd like to get the first and second halves of <code>a</code>, provided <code>a.len()</code> is even.</p>