gh-35564: `sage.misc.misc`, `sage.combinat`: Modularization fixes
    
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### 📚 Description

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- move `powerset` (= `subsets`) from the problematic `sage.misc.misc`
module to `sage.combinat.subset`,
- likewise we move the implementation of `uniq`
(`sage.misc.misc._stable_uniq`) and finish the job from #27014,
- we remove `sage.misc.misc.union`, deprecated in #16604,
- some other fixes to imports.

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Part of:
- #29705
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- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

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- Depends on #35672

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URL: #35564
Reported by: Matthias Köppe
Reviewer(s): David Coudert, Matthias Köppe