NonZero no longer exists

[dtolnay]

$ cargo build --features unstable

error[E0432]: unresolved import `core::nonzero::Zeroable`
   --> serde/src/lib.rs:218:38
    |
218 |     pub use core::nonzero::{NonZero, Zeroable};
    |                                      ^^^^^^^^ no `Zeroable` in `nonzero`

error[E0603]: module `nonzero` is private
   --> serde/src/lib.rs:218:29
    |
218 |     pub use core::nonzero::{NonZero, Zeroable};
    |                             ^^^^^^^

error[E0603]: module `nonzero` is private
   --> serde/src/lib.rs:218:38
    |
218 |     pub use core::nonzero::{NonZero, Zeroable};
    |                                      ^^^^^^^^