Blocking updates

[yoshuawuyts]

Renames task::blocking to task::spawn_blocking, and changes the input type from impl Future to impl FnOnce so that it becomes clear that this function is for non-async code only.

We received public feedback on the signature, and @stjepang and I spoke privately about the method name and agreed that it was indeed confusing with block_on also being a thing.

Because this API has changed, it means that #290 will no longer be able to be called inside task::blocking, but instead tasks should rely on thread::yield_now instead. This overall feels like a positive to me.

Thanks!

[yoshuawuyts]

As per #289, we may want to consider renaming this to task::spawn_thread instead of task::spawn_blocking as to have a counterpart to thread::spawn_task.

[ghost]

Need to think about this.

I'll just throw in another idea for the signature of this function:

pub async fn asyncify<F, R>(f: F)
where
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static,
{
    blocking::spawn(f).await
}

Might also consider putting this function into the async_std::future module.

Unsure about these ideas though. Need to mull over them :)