Implement `Neg` for signed non-zero integers.
Negating a non-zero integer currently requires unpacking to a primitive and re-wrapping. Since negation of non-zero signed integers always produces a non-zero result, it is safe to implement `Neg` for `NonZeroI{N}`.
The new `impl` is marked as stable because trait impls for two stable types can't be marked unstable.
See discussion on https://github.com/rust-lang/libs-team/issues/105 for additional context.
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