title: Is there a std::less/std::greater for the spaceship operator?
body: <p>All the basic comparisons (<code>&lt;</code>, <code>&lt;=</code>, <code>==</code>, <code>!=</code>, <code>&gt;=</code>, <code>&gt;</code>) have an associated function object (<code>std::less</code>, <code>std::less_equal</code>, <code>std::equal_to</code>, <code>std::not_equal_to</code>, <code>std::greater_equal</code>, <code>std::greater</code>).</p>
  <p>Does the spaceship operator <code>&lt;=&gt;</code> have a similar function object?
  If not, why was it not added to the standard library?</p>