feat: change Array.get to take a Nat and a proof

[kim-em]

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.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 258d3725e73688ef08bc410e4cc8ee91a5641f13 --onto 811d8fb3c01cf5724ba4daf733898793cca24bdd. (2024-11-11 10:41:59)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 5c611f78149d9e67f3bff1e3697fb1b504657998 --onto 456e6d2b791bbc560b304b28a60f35db7072c306. (2024-11-11 12:55:08)

[david-christiansen]

One very minor docstring quibble.

It looks like this changeset touches lots of code, and basically all of it gets nicer. That's a nice outcome!