Re-add serde support to NetworkInterface

In #255, the ipnetwork crate was added.
IpNetwork did not yet support Serde, so (de)serialization support using such had to be removed.

As of
achanda/ipnetwork@c9e25e1,
IpNetwork includes feature-gated serde support

This snippet did not function prior to my changes, but seems to function
fine after re-implementing serde support for IpNetwork

```rust
pub fn get_network_interfaces() -> anyhow::Result<Value> {
    let raw_interfaces: Vec<NetworkInterface> = interfaces();
    Ok(json!(raw_interfaces))
}
```

Since this is now supported upstream, it seems like a low-risk change.

Cargo.toml

@@ -17,7 +17,7 @@ netmap = ["pnet_datalink/netmap_sys", "pnet_datalink/netmap"]
 pcap = ["pnet_datalink/pcap"]
 appveyor = []
 travis = []
-serde = ["pnet_base/serde"]
+serde = ["pnet_base/serde", "pnet_datalink/serde"]
 
 [dependencies]
 ipnetwork = "0.16.0"

pnet_datalink/Cargo.toml

@@ -22,5 +22,11 @@ pnet_sys = { path = "../pnet_sys", version = "0.26.0" }
 pcap = { version = "0.7", optional = true }
 netmap_sys = { version = ">=0.0", optional = true, features = ["netmap_with_libs"] }
 
+
+serde = { version = "~1", optional = true, default-features = false }
 [target.'cfg(windows)'.dependencies]
 winapi = "0.3.8"
+
+[package.metadata.docs.rs]
+# Enable the serde feature when generating docs on docs.rs, so the traits are visible
+features = ["serde"]

pnet_datalink/src/lib.rs

@@ -13,6 +13,11 @@ extern crate libc;
 extern crate pnet_base;
 extern crate pnet_sys;
 
+#[cfg(feature = "serde")]
+extern crate serde;
+#[cfg(feature = "serde")]
+use serde::{de, Deserialize, Deserializer, Serialize, Serializer};
+
 use std::io;
 use std::option::Option;
 use std::time::Duration;
@@ -210,6 +215,7 @@ pub trait DataLinkReceiver: Send {
 }
 
 /// Represents a network interface and its associated addresses.
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub struct NetworkInterface {
     /// The name of the interface.