Add a tentative Display trait implementation for ZendObject

Possibly a bad idea as it can panic. Probably best to explicitly cast
before displaying.

src/php/types/object.rs

@@ -3,7 +3,7 @@
 
 use std::{
     convert::TryInto,
-    fmt::Debug,
+    fmt::{self, Debug},
     marker::PhantomData,
     mem::{self, MaybeUninit},
     ops::{Deref, DerefMut},
@@ -238,6 +238,16 @@ impl Debug for ZendObject {
     }
 }
 
+impl fmt::Display for ZendObject {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "{}",
+            self.extract::<String>().expect("Failed to cast to string")
+        )
+    }
+}
+
 /// Wrapper struct used to return a reference to a PHP object.
 pub struct ClassRef<'a, T: RegisteredClass> {
     ptr: &'a mut ZendClassObject<T>,