mac: impl Debug and Display for MacAddress

Adding implementations of the `Debug` and `Display` traits to the
`MacAddress` struct allows creating string representations of these
objects.

src/mac.rs

@@ -37,6 +37,7 @@ impl error::Error for ParseError {
 }
 
 
+#[derive(Debug)]
 pub struct MacAddress {
     addr: [u8; 6],
 }
@@ -67,6 +68,17 @@ impl MacAddress {
 }
 
 
+impl fmt::Display for MacAddress {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let s = self.addr.iter()
+            .map(|b| format!("{:x}", b))
+            .collect::<Vec<_>>()
+            .join(":");
+        write!(f, "{}", s)
+    }
+}
+
+
 #[cfg(test)]
 mod test {
     use super::*;