Implement Eq for Cell and RefCell.

Implement Eq for Cell and RefCell.

`core::cell::Cell<T>` and `core::cell::RefCell<T>` currently implement
`PartialEq` when `T` does, and just defer to comparing `T` values.
There is no reason the same shouldn’t apply to `Eq`.

This enables `#[derive(Eq, PartialEq)]` on e.g.
structs that have a `RefCell` field.