title: std::stoull but with string_view
body: <p>Is there a <code>std::stoull</code> which takes a <code>basic_string_view</code>? I don't want to construct a string just to call <code>std::stoull</code>, especially since it takes it by <code>const &amp;</code>.</p>
  <p>If none exists, what is an efficient way to convert a <code>std::basic_string_view</code> to <code>unsigned long long</code>?</p>