Initialize new T_OBJECT as ROBJECT_EMBED

Previously, when an object is first initialized, ROBJECT_EMBED isn't
set. This means that for brand new objects, ROBJECT_NUMIV(obj) is 0 and
ROBJECT_IV_INDEX_TBL(obj) is NULL.

Previously, this combination meant that the inline cache would never be
initialized when setting an ivar on an object for the first time since
iv_index_tbl was NULL, and if it were it would never be used because
ROBJECT_NUMIV was 0. Both cases always fell through to the generic
rb_ivar_set which would then set the ROBJECT_EMBED flag and initialize
the ivar array.

This commit changes rb_class_allocate_instance to set the ROBJECT_EMBED
flag on the object initially and to initialize all members of the
embedded array to Qundef. This allows the inline cache to be set
correctly on first use and to be used on future uses.

This moves rb_class_allocate_instance to gc.c, so that it has access to
newobj_of. This seems appropriate given that there are other allocating
methods in this file (ex. rb_data_object_wrap, rb_imemo_new).

gc.c

@@ -2373,6 +2373,13 @@ rb_imemo_new_debug(enum imemo_type type, VALUE v1, VALUE v2, VALUE v3, VALUE v0,
 }
 #endif
 
+VALUE
+rb_class_allocate_instance(VALUE klass)
+{
+    VALUE flags = T_OBJECT | ROBJECT_EMBED;
+    return newobj_of(klass, flags, Qundef, Qundef, Qundef, RGENGC_WB_PROTECTED_OBJECT);
+}
+
 VALUE
 rb_data_object_wrap(VALUE klass, void *datap, RUBY_DATA_FUNC dmark, RUBY_DATA_FUNC dfree)
 {

internal/gc.h

@@ -78,6 +78,7 @@ RUBY_ATTR_MALLOC void *rb_xcalloc_mul_add_mul(size_t, size_t, size_t, size_t);
 static inline void *ruby_sized_xrealloc_inlined(void *ptr, size_t new_size, size_t old_size) RUBY_ATTR_RETURNS_NONNULL RUBY_ATTR_ALLOC_SIZE((2));
 static inline void *ruby_sized_xrealloc2_inlined(void *ptr, size_t new_count, size_t elemsiz, size_t old_count) RUBY_ATTR_RETURNS_NONNULL RUBY_ATTR_ALLOC_SIZE((2, 3));
 static inline void ruby_sized_xfree_inlined(void *ptr, size_t size);
+VALUE rb_class_allocate_instance(VALUE klass);
 
 RUBY_SYMBOL_EXPORT_BEGIN
 /* gc.c (export) */

object.c

@@ -2102,13 +2102,6 @@ rb_obj_alloc(VALUE klass)
     return rb_class_alloc(klass);
 }
 
-static VALUE
-rb_class_allocate_instance(VALUE klass)
-{
-    NEWOBJ_OF(obj, struct RObject, klass, T_OBJECT | (RGENGC_WB_PROTECTED_OBJECT ? FL_WB_PROTECTED : 0));
-    return (VALUE)obj;
-}
-
 /*
  *  call-seq:
  *     class.new(args, ...)    ->  obj