Optimize AdditiveMonoids sum() method

[user202729]

Previously the example shown takes way too long. Now it takes 0.5s.

It is probably possible to optimize further by specialize it using Singular.

Opinion: how should the iterator multiplication be implemented

• as is (disadvantage: slightly misleading method name),
• copy paste the code logic of iterator_prod and make a new method iterator_sum (disadvantage: code duplication),
• make an internal method _iterator_combine that takes bint multiply, then make iterator_prod and iterator_sum call it (disadvantage: potential slower branching each operation, but should be negligible)

📝 Checklist

• The title is concise and informative.
• 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.

⌛ Dependencies

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