Add StaticArrayQueue, a queue that can be used as a global static variable

[Finomnis]

Motivation

Currently there is no queue (that I could find) that fulfills all the following criteria:

• no_std compatible (without alloc)
• lock-free
• usable as a static variable

crossbeam_queue::ArrayQueue is very close, but has the following problems:

• Not usable as a static variable (new() function isn't const)
• Requires alloc

Content

This change introduces StaticArrayQueue, which is identical to ArrayQueue except that it contains the array unboxed as a member and has a const constructor.

This allows the following pattern:

use std::{
    thread::{self, sleep},
    time::Duration,
};

use crossbeam_queue::StaticArrayQueue;

static QUEUE: StaticArrayQueue<u32, 10> = StaticArrayQueue::new();

fn main() {
    thread::spawn(|| loop {
        while let Some(element) = QUEUE.pop() {
            println!("Got element: {}", element);
        }
        sleep(Duration::from_millis(1));
    });

    for i in 0..10 {
        QUEUE.push(i).unwrap();
        sleep(Duration::from_millis(10));
    }
}

Open questions/tasks

• Const generics are not supported by Rust 1.38. Bump MSRV? Publish as external crate? (in its current state, the code requires Rust 1.61. So most likely publish this as an external crate)

[Finomnis]

Never mind. It seems like this problem is already solved by heapless::mpmc. Will use that one in my project instead.