code: public <T, A> Function<A, T> getConstructor(Class<? extends T> clazz, Class<A> arg) {
          return getConstructorAs(Function.class, "apply", clazz, arg);
      }
docs: Gets single arg constructor as Function.
  @param clazz class to get constructor for.
  @param arg constructor argument type.
  @param <T> clazz.
  @param <A> argument class.
  @return function.