Implement Display trait for StringRef (#36)

StringRef should implement `Display` trait to get the `to_string` method
rather than implementing `to_string` by itself. Implementing `Display`
is preferred and will make printing StringRef instance easier. Read more
in:
https://rust-lang.github.io/rust-clippy/master/index.html#inherent_to_string

coreaudio-sys-utils/src/string.rs

@@ -44,10 +44,6 @@ impl StringRef {
         Self(string_ref)
     }
 
-    pub fn to_string(&self) -> String {
-        String::from_utf8(utf8_from_cfstringref(self.0)).expect("convert bytes to a String")
-    }
-
     pub fn into_string(self) -> String {
         self.to_string()
     }
@@ -75,6 +71,14 @@ impl Drop for StringRef {
     }
 }
 
+impl std::fmt::Display for StringRef {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let string =
+            String::from_utf8(utf8_from_cfstringref(self.0)).expect("convert bytes to a String");
+        write!(f, "{}", string)
+    }
+}
+
 fn utf8_from_cfstringref(string_ref: CFStringRef) -> Vec<u8> {
     use std::ptr;