add Composer implementation for reference types

The use case is to share the underlying Composer buffer between
builders to avoid allocation, via a thread local variable for example.
Without the reference type implements, the ownership could be
transferred by memory swap operation, but that's not quite ideal. Plus
some methods like MessageBuilder::from_target would drop the Composer
on error, which makes it even difficult to share.

src/base/wire.rs

@@ -46,6 +46,15 @@ impl Composer for bytes::BytesMut {}
 #[cfg(feature = "smallvec")]
 impl<A: smallvec::Array<Item = u8>> Composer for smallvec::SmallVec<A> {}
 
+impl<T: Composer> Composer for &mut T {
+    fn append_compressed_dname<N: ToDname + ?Sized>(
+        &mut self,
+        name: &N,
+    ) -> Result<(), Self::AppendError> {
+        Composer::append_compressed_dname(*self, name)
+    }
+}
+
 //------------ Compose -------------------------------------------------------
 
 /// An extension trait to add composing to foreign types.