NonZero signed variants not available in std

[Mark-Simulacrum]

The NonZero signed variants are not re-exported in std here: https://github.com/rust-lang/rust/blob/master/src/libstd/num.rs#L15

We should duplicate that line and re-export them.

cc @rust-lang/release @rust-lang/libs -- this will likely slip into 1.34, but we will backport the fix into 1.35