Make schedule Send with panics.

[Berrysoft]

However, simply calling wake in other threads may not trigger reschedule. Therefore I don't know how to construct a panic test.

Closes #108 . According to the votes of the maintainers, we don't want to support futures channel working cross thread.

[Berrysoft]

Where is the Send impl

In std:)

You can see the comment of spawn_unchecked:

If schedule is not [Send] and [Sync], all instances of the [Runnable]'s [Waker] must be used and dropped on the original thread.

As we cannot stop the user use and drop the waker on other threads, we should panic to make schedule Send and Sync.

[George-Miao]

Does that mean the old impl we have are unsound?

[Berrysoft]

I think so.