variable.c: add object shapes for MRB_TT_OBJECT IV storage

Introduce "object shapes" (hidden classes) that share IV key
layouts across objects with the same instance variable assignment
order. This eliminates per-object key storage overhead.

Memory savings: ~22% heap reduction for object-heavy workloads
(e.g., 150k objects with 2-6 IVs). Per-object: 40->24 bytes
for 2 IVs. Objects exceeding 16 IVs or using
remove_instance_variable fall back to traditional iv_tbl.

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby/internal.h

@@ -199,6 +199,10 @@ void mrb_gc_free_gv(mrb_state*);
 size_t mrb_gc_mark_iv(mrb_state*, struct RObject*);
 void mrb_gc_free_iv(mrb_state*, struct RObject*);
 
+/* IV shape tree */
+void mrb_init_shape(mrb_state*);
+void mrb_free_shape(mrb_state*);
+
 /* VM */
 #define MRB_CI_VISIBILITY(ci) MRB_FLAGS_GET((ci)->vis, 0, 2)
 #define MRB_CI_SET_VISIBILITY(ci, visi) MRB_FLAGS_SET((ci)->vis, 0, 2, visi)

include/mruby/object.h

@@ -24,6 +24,11 @@ struct RBasic {
 #define MRB_OBJ_IS_FROZEN 1
 #define mrb_frozen_p(o) ((o)->frozen)
 
+/* Object shape flag -- when set, obj->iv is shaped, not iv_tbl* */
+/* Bit 5: avoids conflict with MRB_INSTANCE_TT_MASK (bits 0-4) */
+#define MRB_FL_OBJ_SHAPED (1 << 5)
+#define MRB_OBJ_SHAPED_P(o) ((o)->flags & MRB_FL_OBJ_SHAPED)
+
 struct RObject {
   MRB_OBJECT_HEADER;
   struct iv_tbl *iv;