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>
no need to enter `derive_functional_induction` anymore.
(Will remove the support for `derive_functional_induction` after the
next stage0 update, since we are already using it in Init.)
