feat: change `Array.get` to take a `Nat` and a proof (#6032)

This PR changes the signature of `Array.get` to take a Nat and a proof,
rather than a `Fin`, for consistency with the rest of the (planned)
Array API. Note that because of bootstrapping issues we can't provide
`get_elem_tactic` as an autoparameter for the proof. As users will
mostly use the `xs[i]` notation provided by `GetElem`, this hopefully
isn't a problem.

We may restore `Fin` based versions, either here or downstream, as
needed, but they won't be the "main" functions.

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Co-authored-by: David Thrane Christiansen <david@davidchristiansen.dk>