Add fixed point combinator

[jbgi]

useful for defining recursive functions via lambda syntax.

[jbgi]

Here is what I have in mind:

  static <A> A fix(F<A, A> f, F<F0<A>, A> delay) {
    return new Object() {
      final A a = delay.f(() -> f.f(this.a));
    }.a;
  }

  static <A, B> F<A, B> rec(F<F<A, B>, F<A, B>> f, F<F0<B>, B> delay) {
    return new F<A, B>() {
      final F<A, B> rec = f.f(this);
      @Override
      public B f(A a) {
        return delay.f(() -> rec.f(a));
      }
    };
  }

Does that make sense?