  If the collection does not support an internal
representation in a form of contiguous storage, <code>body</code> is not
called and <code>nil</code> is returned.</p>
<p>A <code>Collection</code> that provides its own implementation of this method
must also guarantee that an equivalent buffer of its <code>SubSequence</code>
can be generated by advancing the pointer by the distance to the
slice's <code>startIndex</code>.</p>
<h4>Declaration</h4><code class="language-swift">@inlinable public func withContiguo