fix(tactic/norm_num) Bugfix for norm num when testing divisibility of integers

[alexjbest]

They were assumed nats somehow leading to

example (h : (5 : ℤ) ∣ 2) : false := by norm_num at h

giving

type mismatch at application
  has_mod.mod 2
term
  2
has type
  ℤ
but is expected to have type
  ℕ

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 the tactic doc entries
  • write an example of use of the new feature in test/tactics.lean or another test file
• 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