Add support for empty immutable shared arrays

Why:

- Since PHP 7.3, it's possible for extensions to create zvals backed by
  an immutable shared hashtable via the ZVAL_EMPTY_ARRAY macro.
- This helps avoid redundant hashtable allocations when returning empty arrays
  back to userland PHP code, and could likewise be beneficial for Rust extensions too.

What:

- Add ZendHashTable::new_empty_immutable() to obtain a ZendHashTable
  that is actually an empty immutable shared hashtable.
- Add ZendHashTable::is_immutable(). Use it to avoid attempting to
  free the immutable shared hashtable on drop, and to set appropriate
  type flags when initializing a zval with a ZendHashTable.
- Make ZendHashTable's TryFrom implementations from Vec and HashMap return
  an empty immutable shared hashtable if the input collection was empty.
  Although this would allow every user to automatically benefit from
  this optimization, I'm not convinced about this part because this is technically a
  breaking change for consumers that construct ZendHashTables via these
  converters in their Rust code. Maybe it'd be better to not change the
  converters and let projects explicitly use ::new_empty_immutable if they
  deem the optimization would be useful.

Author: mszabo-wikia

allowed_bindings.rs

@@ -81,6 +81,7 @@ bind! {
     zend_declare_class_constant,
     zend_declare_property,
     zend_do_implement_interface,
+    zend_empty_array,
     zend_execute_data,
     zend_function_entry,
     zend_hash_clean,
@@ -137,6 +138,9 @@ bind! {
     E_RECOVERABLE_ERROR,
     E_DEPRECATED,
     E_USER_DEPRECATED,
+    GC_IMMUTABLE,
+    GC_FLAGS_MASK,
+    GC_FLAGS_SHIFT,
     HT_MIN_SIZE,
     IS_ARRAY,
     IS_ARRAY_EX,