towards allowing to merge polynomials rings

[fchapoton]

The aim would be to allow adding elements of polynomials rings with different variables.

For instance

sage: x,y=PolynomialRing(ZZ,'x,y').gens()
sage: u,v=PolynomialRing(ZZ,'u,v').gens()
sage: z=polygen(QQbar,'z')
sage: t=PolynomialRing(QQ,'t').gen()
sage: x+u
u + x
sage: x+z
z + x
sage: t+z
z + t
sage: t+x
x + t

This is done by refactoring the merge and __mul__ methods in PolynomialFunctor and MultiPolynomialFunctor

This seems to have side effects on the infinite polynomial rings.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• 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 accordingly.

⌛ Dependencies

[fchapoton]

87 related failing doctests, but most are improvements (success replacing failure)

[github-actions]

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