feat: add difference on ExtDHashMap/ExtHashMap/ExtHashSet

[wkrozowski]

This PR adds support for the difference operation for ExtDHashMap/ExtHashMap/ExtHashSet and proves several lemmas about it.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2025-11-27 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2025-11-27 13:45:46)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase e548fa414cb74db21a31afe6c26cb0c51310f1db --onto 0173444d24df9253d7947cfe82519e4a6caffd3a. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-04 13:04:32)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2025-11-27 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2025-11-27 13:45:48)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase e548fa414cb74db21a31afe6c26cb0c51310f1db --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-12-04 13:04:34)

[TwoFX]

@wkrozowski The GitHub diff seems to be confused here; would you mind merging master into your branch so that we don't see changes that are already merged?

[wkrozowski]

@TwoFX Okay, done