code: public JSON with(Feature feature, boolean state)
      {
          int f = _features;
          if (state) {
              f |= feature.mask();
          } else {
              f &= ~feature.mask();
          }
          return _with(f);
      }
docs: Mutant factory for constructing an instance with specified feature
  enabled or disabled (depending on <code>state</code>), and returning
  an instance with that setting; this may either be this instance (if feature
  already had specified state), or a newly constructed instance.