adding value versions for is_true/is_false

Hi, would you consider adding value versions of the is_true/is_false functions so you can write code like

template<class T>
auto requires_(T) -> tick::valid<
  is_true_c<X == Y>
>;

(using the "_c" naming convention from boosts mpl library http://www.boost.org/doc/libs/1_56_0/libs/mpl/)