Refactor spin and spin_once to mimic rclcpp's Executor API

[esteve]

Moving spin and spin_once to a similar API as rclcpp's and rclpy's Executor API allows us to run more than one node in a single program and makes it easier to test node behaviors.

[esteve]

@jhdcs @maspe36 these changes are functionally equivalent to what we alredy have now, but add the extra feature of being able to run more than one node in a program (see the example I've added where there's a publisher and two subscriptions). I plan on expanding this to enable multiple implementations of an executor following rclcpp/rclpy's approach, but for now I just wanted to get a concrete implementation and then I'll derive traits and other generalizations in a follow-up PR.

This would also allow us to implement composable nodes in the future, but for now the spin method in SingleThreadedExecutor just takes an &Arc<Node>

[jhdcs]

Fantastic work!

I only have a couple very-small issues with the comments for spin in lib.rs. Of the two, I think the potentially-unneeded SAFETY comment is the more important to take a look at, but I still wanted to call out a potential inaccuracy.

But all in all, this is beautifully done, thank you!

[esteve]

@jhdcs thanks for the review!

You're right about the comments, I'll update/remove them.

But all in all, this is beautifully done, thank you!

These are very kind words, I really appreciate it, thanks 🙂

[esteve]

@jhdcs I've addressed your feedback, could you have another look when you have a moment? Thanks!

[jhdcs]

Looks good to me! Thank you again for this!