[Merged by Bors] - feat(Algebra/Order/Field/Basic): add Nat cast lemmas

[mariainesdff]

These lemmas have one-line proofs, but I use them enough times in #15373 that I think it makes sense to introduce them.

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[github-actions]

PR summary a28a96dda8

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ one_div_cast_ne_zero
+ one_div_cast_nonneg
+ one_div_cast_pos

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

[j-loreaux]

I think these belong in Mathlib.Data.Nat.Cast.Order.Field

bors d+

[mathlib-bors]

✌️ mariainesdff can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[mariainesdff]

bors r+

[mathlib-bors]

Pull request successfully merged into master.

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