When embedded strings are moved, not all the embedded bytes are copied over to the new embedded string. This shows itself when
an embedded string has more than 16 characters.
rb_obj_alloc(RBASIC_CLASS(obj)) will always allocate from the basic
40B pool, so if obj is larger than 40B, we'll create a corrupted
object when we later copy the shape_id.
Instead we can use the same logic than ractor copy, which is
to use rb_obj_clone, and later ask the GC to free the original
object.
We then must turn it into a T_OBJECT, because otherwise
just changing its class to RactorMoved leaves a lot of
ways to keep using the object, e.g.:
a = [1, 2, 3]
Ractor.new{}.send(a, move: true)
[].concat(a) # Should raise, but wasn't.
If it turns out that rb_obj_clone isn't performant enough
for some uses, we can always have carefully crafted specialized
paths for the types that would benefit from it.
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