NonZero can no longer be dereferenced

[dtolnay]

$ cargo build --features unstable
   Compiling serde v1.0.2
error: type `core::nonzero::NonZero<T>` cannot be dereferenced
   --> src/ser/impls.rs:328:17
    |
328 |                 (**self).serialize(serializer)
    |                 ^^^^^^^^
...
350 | deref_impl!(<T> Serialize for NonZero<T> where T: Serialize + Zeroable);
    | ------------------------------------------------------------------------ in this macro invocation

[dtolnay]

Looks like rust-lang/rust#41064.