Reimplement 'parent' construct as variable $parent.

This commit also adds new controls to variables, which prevents from re-assigning a value when set. Thanks to that, both $this and $parent cannot be overwritten by using OP_STORE instruction. Other instructions still need some
work.

include/ph7int.h

@@ -643,6 +643,9 @@ struct ph7_value {
 #define MEMOBJ_MIXED     0x0200  /* Memory value is mixed */
 #define MEMOBJ_HASHMAP   0x0400  /* Memory value is a hashmap aka 'array' in the PHP jargon */
 #define MEMOBJ_NULL      0x0800  /* Memory value is NULL */
+/* Special purpose */
+#define MEMOBJ_FIXEDVAL  0x1000  /* Memory value is fixed and cannot be modified */
+#define MEMOBJ_PARENTOBJ 0x2000  /* Memory value is a parent object */
 /* Mask of all known types */
 #define MEMOBJ_ALL (MEMOBJ_STRING|MEMOBJ_INT|MEMOBJ_REAL|MEMOBJ_BOOL|MEMOBJ_NULL|MEMOBJ_HASHMAP|MEMOBJ_OBJ|MEMOBJ_RES|MEMOBJ_CALL|MEMOBJ_CHAR|MEMOBJ_VOID)
 /* Scalar variables