Remove `PhantomData<*mut Waiter>` so we don't need to manually `impl Send`

[Enselic]

The effect of having PhantomData<*mut Waiter> is that the OnceCell<T> stops being impl<T: Send> Send for OnceCell<T>. But we want it to be impl<T: Send> Send for OnceCell<T>, because we manually mark it as such.

By removing the PhantomData we can also remove the manual impl Send. The net effect is zero changes to the public API surface (including auto traits) of once_cell:

$ cargo install cargo-public-api --locked
$ cargo public-api diff origin/master..remove-unneeded-phantomdata
Removed items from the public API
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(none)

Changed items in the public API
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(none)

Added items to the public API
=============================
(none)

A variant of this PR is to remove the PhantomData but keep the manual impl Send. Maybe you prefer the explicitness.

Background of this PR

I am studying how once_cell is implemented, and could not at first understand why we needed the PhantomData<*mut Waiter>. Digging deeper, I concluded that we do not in fact need it.

[matklad]

Yeah, phantom data is no longer needed, as we switched to AtomicPtr a while ago. Let’s keep the explicit Send bound here — send/sync bounds are versus subtle here, so better to spell them out explicitly

[matklad]

Let me merge as is while I am at a laptop..