perf(data/gaussian_int): speed up div and mod

[ChrisHughes24]

avoid using int.cast, and use rat.of_int.

This sped up #eval (⟨1414,152⟩ : gaussian_int) % ⟨123,456⟩ from about 5 seconds to 2 milliseconds

TO CONTRIBUTORS:

Make sure you have:

• reviewed and applied the coding style: coding, naming
• reviewed and applied the documentation requirements
• for tactics:
  • added or adapted documentation in tactics.md
  • write an example of use of the new feature in tactics.lean
• make sure definitions and lemmas are put in the right files
• make sure definitions and lemmas are not redundant

If this PR is related to a discussion on Zulip, please include a link in the discussion.

For reviewers: code review check list