Implement Display trait for CompressionMethod.

It's an alias to the Debug format, but we can't derive Display yet,
so we associate it explicitly.

src/compression.rs

@@ -1,5 +1,7 @@
 //! Possible ZIP compression methods.
 
+use std::fmt;
+
 /// Compression methods for the contents of a ZIP file.
 #[derive(Copy, Clone, PartialEq, Debug)]
 pub enum CompressionMethod
@@ -39,6 +41,13 @@ impl CompressionMethod {
     }
 }
 
+impl fmt::Display for CompressionMethod {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        // Just duplicate what the Debug format looks like, i.e, the enum key:
+        write!(f, "{:?}", self)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::CompressionMethod;