Add PYSTON_CONST macro

For some struct member in Pyston itself, add `const` qualifier,
for exenstion, do not add `const` for compatibility reason.

from_cpython/Include/object.h

@@ -190,8 +190,7 @@ typedef struct bufferinfo {
                              pointed to by strides in simple case.*/
     int readonly;
     int ndim;
-    // Pyston change: changed this from char* to const char*
-    const char *format;
+    PYSTON_CONST char *format;
     Py_ssize_t *shape;
     Py_ssize_t *strides;
     Py_ssize_t *suboffsets;

from_cpython/Include/pyport.h

@@ -16,6 +16,12 @@
 #define PYSTON_NOEXCEPT
 #endif
 
+#ifdef _PYSTON_API
+#define PYSTON_CONST const
+#else
+#define PYSTON_CONST
+#endif
+
 #define Py_PROTO(x) x
 
 // Pyston change: these are just hard-coded for now: