Change return type of Ipv4Network::size to u32

Also, generalize size over IpvNetwork
NOTE: this is a breaking change

src/common.rs

@@ -1,6 +1,5 @@
 use std::error::Error;
 use std::fmt;
-use std::net::Ipv4Addr;
 
 /// Represents a bunch of errors that can occur while working with a `IpNetwork`
 #[derive(Debug, Clone, PartialEq, Eq)]