[Merged by Bors] - feat: lemmas on ordinal Nat.cast

[vihdzp]

Downstreamed from the CGT repo.

---

[github-actions]

PR summary 4ec5d95aa6

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ eq_natCast_of_le_natCast
+ eq_natCast_or_omega0_le
+ natCast_image_Iio

You can run this locally as follows

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

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

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

---

No changes to technical debt.

You can run this locally as

./scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[b-mehta]

Note there's a merge conflict.
Other than that, and the comment above, looks good.

[mathlib-merge-conflicts]

This pull request has conflicts, please merge master and resolve them.

[b-mehta]

Thanks!

bors merge

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

• ci (staging) / Post-CI job
• ci (staging) / Lint style
• ci (staging) / Post-Build Step
• ci (staging) / Build