impl Serialize and Deserialize for std::num::NonZero*

… gated on the `unstable` Cargo feature.

These are new standard library types.

serde/src/ser/mod.rs

@@ -94,6 +94,7 @@
 //!    - PathBuf
 //!    - Range\<T\>
 //!    - NonZero\<T\> (unstable)
+//!    - num::NonZero* (unstable)
 //!  - **Net types**:
 //!    - IpAddr
 //!    - Ipv4Addr