An WIP implementation of behavior trees using continuations designed to work with asio

the behavior is a smart pointer wrapping an object satisfying one of these requirements.

• is invocable as status()
  • used for leaf nodes that only continues::up the tree

    behavior x = []{ do_stuff(); return status::success; };

• is invocable as void(continuation)
  • used for leaf nodes that continues::elsewhere, that is it moves the continuation to be called later, for async processing

    behavior x = [](continuation c)
    {
        timer t(some_time);
        t.async_wait([](...){ do_stuff(); c(); });
    };

• is invocable as continues(continuation)
  • used for nodes which continues::down the tree, as well as either of the above usages

    node::operator(continuation r) -> continues
    {
        resume_parent = std::move(r);
        resume_this = [this](status s) -> continues
        {
            // random example
            switch (state()) {
            case 0: return continues::down(child, resume_this);
            case 1: return continues::up(std::move(resume_parent), Success);
            case 2: post(std::move(resume_parent));
                    return continues::elsewhere();
            }
        }
        return continues::down(child, resume_this);
    }

Unlike regular behavoir trees, there is no Running state due to the continuations, since we simply continue the tree by calling them once we finish a task.