Initialize ivars to nil

This branch initializes all ivar's to `nil`. By doing this we create
less new shapes becasue the ivar order is already set. This branch is
experimental. One caveat is this will break code that checks if an
instance variable is defined because it will always be
"instance-variable" after this change.

gc.c

@@ -2873,6 +2873,39 @@ rb_obj_embedded_size(uint32_t numiv)
     return offsetof(struct RObject, as.ary) + (sizeof(VALUE) * numiv);
 }
 
+static enum rb_id_table_iterator_result
+method_entry_i(ID key, VALUE value, void *data)
+{
+    const rb_method_entry_t *me = (const rb_method_entry_t *)value;
+    int type = me->def->type;
+
+    if (type == VM_METHOD_TYPE_IVAR || type == VM_METHOD_TYPE_ATTRSET) {
+        ID mid = me->def->body.attr.id;
+        VALUE instance = (VALUE) data;
+        rb_ivar_set(instance, mid, Qnil);
+    }
+
+    return ID_TABLE_CONTINUE;
+}
+
+static void
+add_instance_method_list(VALUE mod, VALUE instance)
+{
+    struct rb_id_table *m_tbl = RCLASS_M_TBL(mod);
+    if (!m_tbl) return;
+    rb_id_table_foreach(m_tbl, method_entry_i, (void *) instance);
+}
+
+static void
+initialize_default_ivs(VALUE klass, VALUE instance) {
+    VALUE super = RCLASS_SUPER(klass);
+    if (super) {
+        initialize_default_ivs(super, instance);
+    }
+
+    add_instance_method_list(klass, instance);
+}
+
 static VALUE
 rb_class_instance_allocate_internal(VALUE klass, VALUE flags, bool wb_protected)
 {
@@ -2895,6 +2928,8 @@ rb_class_instance_allocate_internal(VALUE klass, VALUE flags, bool wb_protected)
     // SIZE_POOL_COUNT away from the root shape.
     ROBJECT_SET_SHAPE_ID(obj, ROBJECT_SHAPE_ID(obj) + SIZE_POOL_COUNT);
 
+    initialize_default_ivs(klass, obj);
+
 #if RUBY_DEBUG
     RUBY_ASSERT(!rb_shape_obj_too_complex(obj));
     VALUE *ptr = ROBJECT_IVPTR(obj);