task + async misc

[d3zd3z]

Create a new proc-macro-based way of declaring threads in Zephyr. Instead of the weird kobj_define macro, provide a zephyr::thread macro that can decorate an ordinary Rust function to become a builder for a ReadyThread that can just be started. Threads can also now be reused after they exit.

This allows something like:

#[zephyr::thread(stack_size = 2048)]
fn mythread(name: &'static str, index: usize) {
    // Body
}

and somewhere in the code, you just call it, which returns a handle that can be used to start the thread.

let joiner = mythread("thename", 42).start();
...
joiner.join();  // Wait for thread to exit, if desired.

There is still some things to clean up, hence the draft, but I wanted to get some exposure.

[d3zd3z]

I'd like to start getting this ready for review. But, there is a question of how much this should be split apart. There are a few cherry-picked changes that are to be merged, so we'll at least wait for those to go in.