 function for 16-character digest, with the same
    specification as <a href="Pervasives.html#VALcompare"><code class="code">compare</code></a> and the implementation
    shared with <a href="String.html#VALcompare"><code class="code"><span class="constructor">String</span>.compare</code></a>. Along with the type <code class="code">t</code>, this
    function <code class="code">compare</code> allows the module <code class="code"><span class="constructor">Digest</span></code> to be passed as
    argument to