Add negation methods for signed non-zero integers.
Performing negation with defined wrapping semantics (such as `wrapping_neg()`) on 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 the various `*_neg()` methods for `NonZeroI{N}`.
I'm not sure what to do about the `#[unstable(..., issue = "none")]` here -- should I file a tracking issue, or is that handled by the Rust dev team?
ACP: https://github.com/rust-lang/libs-team/issues/105
609152aa8a