Consider using a *mut () to remove Send and Sync

[agashlin]

In a few places I've seen a *mut () used to make a struct without Send and Sync traits, for instance this:

// We store a raw pointer because it's the only way at the moment to remove `Send`/`Sync` from the
// object.
struct ComInitialized(*mut ());

Currently I'm using PhantomData<Rc<()>> for the same:

comedy-rs/src/com.rs

Lines 142 to 148 in c244b91

	pub struct ComApartmentScope {
	/// PhantomData used in lieu of unstable impl !Send + !Sync.
	/// It must be dropped on the same thread it was created on so it can't be Send,
	/// and references are meant to indicate that COM has been inited on the current thread so it
	/// can't be Sync.
	_do_not_send: PhantomData<Rc<()>>,
	}

I think PhantomData is still the right way to go, but it may be clearer to do PhantomData<*mut ()>. I don't know if there is any practical difference.

Also using a tuple struct instead of one unused named field would be less noisy.

While I'm at it, there should be a compile_fail doctest to ensure Send and Sync are missing (I don't know of any other way to assert a negative impl).