Consider adding CellWriter, CellReader ZSTs

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Consider adding:

• struct CellWriter<'a, F: Family>(PhantomData<&'a mut ()>)
• struct CellReader<'a, F: Family>(PhantomData<&'a ()>)

The former defines mutation functions, as well as borrow_mut(&'b mut self) -> CellWriter<'b, F> and borrow(&'b self) -> CellReader<'a, F>. The latter defines read functions as well and is Copyable. Thanks to their lifetime parameters and a careful assignment of Copy / Send / Sync implementations, they could provide the same guarantees as &mut CellOwner<F> and &CellOwner<F> while being zero-sized.

Though &mut CellOwner / &CellOwner (with CellOwner a ZST) can be well optimized, they are nonetheless not zero-sized, so such a change would make borrowing a CellOwner completely free.