Unified spawning

[NthTensor]

This unifies ThreadPool::spawn, ThreadPool::spawn_future and ThreadPool::spawn_async into a single generic ThreadPool::spawn function, and does the same for Scope::spawn, Scope::spawn_future and Scope::spawn_async.

This brings the api closer to bevy_tasks (which only supports async spawns) without sacrificing the rayon-style blocking spawns.

All of these are now valid:

// Spawn a blocking task
THREAD_POOL.spawn(|| {
    for i in 0..100 {
        blocking_work(i);
    }
});

// Spawn a future
let task = THREAD_POOL.spawn(async {
    let mut result = None;
    for i in 0..100 {
        result = async_work(i).await;
    }
    result
});

// Spawn an async function
THREAD_POOL.spawn(do_something).detach();

async fn do_something() {
    for i in 0..100 {
        result = async_work(i).await;
    }
}

[NthTensor]

I'm going to merge these changes as part of #40.