enable PyStringObject: we use now the same layout as cpython

from_cpython/Include/stringobject.h

@@ -33,8 +33,6 @@ functions should be applied to nil objects.
    interning of any string.
    Together, these sped the interpreter by up to 20%. */
 
-// Pyston change: comment this out since this is not the format we're using
-#if 0
 typedef struct {
     PyObject_VAR_HEAD
     long ob_shash;
@@ -50,9 +48,6 @@ typedef struct {
      *       from 'interned' to this object are *not counted* in ob_refcnt.
      */
 } PyStringObject;
-#endif
-struct _PyStringObject;
-typedef struct _PyStringObject PyStringObject;
 
 #define SSTATE_NOT_INTERNED 0
 #define SSTATE_INTERNED_MORTAL 1

src/runtime/types.h

@@ -408,7 +408,8 @@ public:
     llvm::StringRef s() const { return llvm::StringRef(s_data, ob_size); };
 
     long hash; // -1 means not yet computed
-    char interned_state;
+    int interned_state;
+    char s_data[0];
 
     char* data() { return s_data; }
     const char* c_str() {
@@ -463,10 +464,13 @@ private:
 
     BoxedString(size_t n); // non-initializing constructor
 
-    char s_data[0];
-
     friend void setupRuntime();
 };
+static_assert(sizeof(BoxedString) == sizeof(PyStringObject), "");
+static_assert(offsetof(BoxedString, ob_size) == offsetof(PyStringObject, ob_size), "");
+static_assert(offsetof(BoxedString, hash) == offsetof(PyStringObject, ob_shash), "");
+static_assert(offsetof(BoxedString, interned_state) == offsetof(PyStringObject, ob_sstate), "");
+static_assert(offsetof(BoxedString, s_data) == offsetof(PyStringObject, ob_sval), "");
 
 extern "C" size_t strHashUnboxed(BoxedString* self);
 extern "C" int64_t hashUnboxed(Box* obj);