Not every caller (for example, YJIT) actually needs to pass the object.
YJIT (and, in the future, ZJIT) only need to pass the class.

This should never be true. I added an `rb_bug` in case it was and it
wasn't true in any of btest or test-all.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

[Bug #21710]

- struct.c: `struct_alloc`

It is possible for a `NEWOBJ` tracepoint call back to write fields
into a newly allocated object before `struct_alloc` had the time
to set the `RSTRUCT_GEN_FIELDS` flags and such.

Hence we can't blindly initialize the `fields_obj` reference to `0`
we first need to check no fields were added yet.

- object.c: `rb_class_allocate_instance`

Similarly, if a `NEWOBJ` tracepoint tries to set fields on the object,
the `shape_id` must already be set, as it's required on T_OBJECT to
know where to write fields.

`NEWOBJ_OF` had to be refactored to accept a `shape_id`.

`rb_ivar_delete` would force a new shape to be created.
And in the case of a shape exhaustion, it wouldn't handle
T_CLASS/T_MODULE correctly.

The embed layout is way more common than the heap one,
especially since WVA.

I think it makes for more readable code to inverse the
flag.

Its usage was removed in 306d50811dd060d876d1eb364a0d5e6106f5e4f1.

It only makes sense for heap objects.