gh-38987: some work on deformed Burau matrix

    
some refactoring work on deformed Burau matrix

to try to make it faster

(this is about investigating #38869)

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.
    
URL: #38987
Reported by: Frédéric Chapoton
Reviewer(s): Travis Scrimshaw