Change ArcWake::into_waker to a free function

[taiki-e]

I feel this to be more consistent.

pub fn waker<W>(wake: Arc<W>) -> Waker where W: ArcWake;

pub fn waker_ref<W>(wake: &Arc<W>) -> WakerRef<'_> where W: ArcWake;

[cramertj]

Part of the reason for doing it this way was to make the method more discoverable. I don't care particularly strongly about this change either way, but I think we need some extra documentation on the trait if we're going to make this a free function.

[taiki-e]

@cramertj Sorry for the late reply. I added documents for ArcWake -> Waker conversion in 1a3e8a4.

[cramertj]

Works for me, then!