Remove `Self: NoCell` bound from `AsBytes::write_to`

[joshlf]

In #682, we add a Self: NoCell bound to AsBytes::write_to and similar methods. This is technically unnecessary since we should be able to read the bytes of &self in the same way that an &UnsafeCell can be read, but removing the bound will require some manual reimplementation and careful reasoning about soundness.

[joshlf]

Not sure what I was thinking here, but I'm pretty sure there's no way to do this soundly.