Added iter function on ZendHashTable

We cannot implement `IntoIterator` for the `ZendHashTable` object as we
do not manage the memory, therefore we don't actually 'consume' the
object as the trait intends. Therefore, `IntoIterator` is implemented on
`&'a ZendHashTable`. This `ZendHashTable::iter` function simply forwards
the call.

Author: davidcole1340

src/php/types/array.rs

@@ -233,6 +233,12 @@ impl ZendHashTable {
         unsafe { zend_hash_next_index_insert(self.ptr, Box::into_raw(Box::new(val))) };
     }
 
+    /// Returns an iterator over the hash table.
+    #[inline]
+    pub fn iter(&self) -> Iter<'_> {
+        self.into_iter()
+    }
+
     /// Converts the hash table into a raw pointer to be passed to Zend.
     pub(crate) fn into_ptr(mut self) -> *mut HashTable {
         self.free = false;