Mark `ffi::ArrowArray::try_new` is safe

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

ffi::ArrowArray::try_new is marked unsafe, this is despite being called by the safe function Array::to_raw.

I initially thought this was a mistake in Array::to_raw but I think this API shouldn't actually be marked unsafe. In particular leaking memory is not unsafe, and this appears to be the only consequence from misuse.

Describe the solution you'd like

We should make ffi::ArrowArray::try_new safe

Describe alternatives you've considered

We could not do this

Additional context