[#153] Add optional JSONSchema implementation for IpNetwork

Problem: No instance of `JSONSchema` is implemented for
`IpNetwork`.

Solution: Derive implementation via `schemars`. Add corresponding
feature to the crate.

src/ipv4.rs

@@ -5,6 +5,7 @@ const IPV4_BITS: u8 = 32;
 
 /// Represents a network range where the IP addresses are of v4
 #[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct Ipv4Network {
     addr: Ipv4Addr,
     prefix: u8,