Make Namespace::Root TypedData inherit from Namespace::Entry

The two types of TypedData objects should be treated the same except for
the free function. Since Namespace::Root did not inherit from Namespace::Entry,
all the TypedData_Get_Struct calls for Namespace::Root would raise
"wrong argument type Namespace::Root (expected Namespace::Entry)".

Author: peterzhu2118

namespace.c

@@ -268,7 +268,7 @@ const rb_data_type_t rb_root_namespace_data_type = {
         namespace_entry_memsize,
         rb_namespace_gc_update_references,
     },
-    0, 0, RUBY_TYPED_FREE_IMMEDIATELY // TODO: enable RUBY_TYPED_WB_PROTECTED when inserting write barriers
+    &rb_namespace_data_type, 0, RUBY_TYPED_FREE_IMMEDIATELY // TODO: enable RUBY_TYPED_WB_PROTECTED when inserting write barriers
 };
 
 VALUE