constify again.

Same as last commit, make some fields `const`.

include/ruby/ruby.h:
* Rasic::klass
* RArray::heap::aux::shared_root
* RRegexp::src
internal.h:
* rb_classext_struct::origin_, redefined_class
* vm_svar::cref_or_me, lastline, backref, others
* vm_throw_data::throw_obj
* vm_ifunc::data
* MEMO::v1, v2, u3::value

While modifying this patch, I found write-barrier miss on
rb_classext_struct::redefined_class.

Also vm_throw_data::throw_state is only `int` so change the type.

class.c

@@ -176,7 +176,7 @@ class_alloc(VALUE flags, VALUE klass)
      */
     RCLASS_SET_ORIGIN((VALUE)obj, (VALUE)obj);
     RCLASS_SERIAL(obj) = rb_next_class_serial();
-    RCLASS_REFINED_CLASS(obj) = Qnil;
+    RB_OBJ_WRITE(obj, &RCLASS_REFINED_CLASS(obj), Qnil);
     RCLASS_EXT(obj)->allocator = 0;
 
     return (VALUE)obj;

eval.c

@@ -1356,7 +1356,7 @@ rb_using_refinement(rb_cref_t *cref, VALUE klass, VALUE module)
     FL_SET(module, RMODULE_IS_OVERLAID);
     superclass = refinement_superclass(superclass);
     c = iclass = rb_include_class_new(module, superclass);
-    RCLASS_REFINED_CLASS(c) = klass;
+    RB_OBJ_WRITE(c, &RCLASS_REFINED_CLASS(c), klass);
 
     RCLASS_M_TBL(OBJ_WB_UNPROTECT(c)) =
       RCLASS_M_TBL(OBJ_WB_UNPROTECT(module)); /* TODO: check unprotecting */
@@ -1365,8 +1365,8 @@ rb_using_refinement(rb_cref_t *cref, VALUE klass, VALUE module)
     while (module && module != klass) {
 	FL_SET(module, RMODULE_IS_OVERLAID);
 	c = RCLASS_SET_SUPER(c, rb_include_class_new(module, RCLASS_SUPER(c)));
-	RCLASS_REFINED_CLASS(c) = klass;
-	module = RCLASS_SUPER(module);
+        RB_OBJ_WRITE(c, &RCLASS_REFINED_CLASS(c), klass);
+        module = RCLASS_SUPER(module);
     }
     rb_hash_aset(CREF_REFINEMENTS(cref), klass, iclass);
 }
@@ -1451,12 +1451,12 @@ add_activated_refinement(VALUE activated_refinements,
     FL_SET(refinement, RMODULE_IS_OVERLAID);
     superclass = refinement_superclass(superclass);
     c = iclass = rb_include_class_new(refinement, superclass);
-    RCLASS_REFINED_CLASS(c) = klass;
+    RB_OBJ_WRITE(c, &RCLASS_REFINED_CLASS(c), klass);
     refinement = RCLASS_SUPER(refinement);
     while (refinement && refinement != klass) {
 	FL_SET(refinement, RMODULE_IS_OVERLAID);
 	c = RCLASS_SET_SUPER(c, rb_include_class_new(refinement, RCLASS_SUPER(c)));
-	RCLASS_REFINED_CLASS(c) = klass;
+        RB_OBJ_WRITE(c, &RCLASS_REFINED_CLASS(c), klass);
 	refinement = RCLASS_SUPER(refinement);
     }
     rb_hash_aset(activated_refinements, klass, iclass);

include/ruby/ruby.h

@@ -887,7 +887,7 @@ enum ruby_fl_type {
 
 struct RUBY_ALIGNAS(SIZEOF_VALUE) RBasic {
     VALUE flags;
-    VALUE klass;
+    const VALUE klass;
 };
 
 VALUE rb_obj_hide(VALUE obj);
@@ -1054,7 +1054,7 @@ struct RArray {
 	    long len;
 	    union {
 		long capa;
-                VALUE shared_root;
+                const VALUE shared_root;
 	    } aux;
 	    const VALUE *ptr;
 	} heap;
@@ -1109,7 +1109,7 @@ struct RArray {
 struct RRegexp {
     struct RBasic basic;
     struct re_pattern_buffer *ptr;
-    VALUE src;
+    const VALUE src;
     unsigned long usecnt;
 };
 #define RREGEXP_PTR(r) (RREGEXP(r)->ptr)

internal.h

@@ -1009,8 +1009,8 @@ struct rb_classext_struct {
      */
     rb_subclass_entry_t **module_subclasses;
     rb_serial_t class_serial;
-    VALUE origin_;
-    VALUE refined_class;
+    const VALUE origin_;
+    const VALUE refined_class;
     rb_alloc_func_t allocator;
 };
 
@@ -1127,10 +1127,10 @@ imemo_type_p(VALUE imemo, enum imemo_type imemo_type)
 /*! SVAR (Special VARiable) */
 struct vm_svar {
     VALUE flags;
-    VALUE cref_or_me; /*!< class reference or rb_method_entry_t */
-    VALUE lastline;
-    VALUE backref;
-    VALUE others;
+    const VALUE cref_or_me; /*!< class reference or rb_method_entry_t */
+    const VALUE lastline;
+    const VALUE backref;
+    const VALUE others;
 };
 
 
@@ -1140,9 +1140,9 @@ struct vm_svar {
 struct vm_throw_data {
     VALUE flags;
     VALUE reserved;
-    VALUE throw_obj;
+    const VALUE throw_obj;
     const struct rb_control_frame_struct *catch_frame;
-    VALUE throw_state;
+    int throw_state;
 };
 
 #define THROW_DATA_P(err) RB_TYPE_P((VALUE)(err), T_IMEMO)
@@ -1163,7 +1163,7 @@ struct vm_ifunc {
     VALUE flags;
     VALUE reserved;
     VALUE (*func)(ANYARGS);
-    void *data;
+    const void *data;
     struct vm_ifunc_argc argc;
 };
 
@@ -1220,12 +1220,12 @@ void rb_strterm_mark(VALUE obj);
 struct MEMO {
     VALUE flags;
     VALUE reserved;
-    VALUE v1;
-    VALUE v2;
+    const VALUE v1;
+    const VALUE v2;
     union {
 	long cnt;
 	long state;
-        VALUE value;
+        const VALUE value;
 	VALUE (*func)(ANYARGS);
     } u3;
 };

vm_insnhelper.h

@@ -203,7 +203,7 @@ static inline int
 THROW_DATA_STATE(const struct vm_throw_data *obj)
 {
     VM_ASSERT(THROW_DATA_P(obj));
-    return (int)obj->throw_state;
+    return obj->throw_state;
 }
 
 static inline int
@@ -224,7 +224,7 @@ static inline void
 THROW_DATA_STATE_SET(struct vm_throw_data *obj, int st)
 {
     VM_ASSERT(THROW_DATA_P(obj));
-    obj->throw_state = (VALUE)st;
+    obj->throw_state = st;
 }
 
 static inline void