Use rust style instead of old-fashioned loops

[emmorais]

New algorithms for polynomial multiplications and inverses were implemented using old-fashioned loops. The pros and cons of using this approach could lead to a long discussion. Being more pragmatic, we are primarily interested in performance, therefore if using the appropriate rust techniques leads to better performance, we must use it. Even if performance is same, we should opt for the rust style.

• [ ] Benchmark and compare the practical effect of different possible refactorings.
• [ ] Decide for the best solution and apply it.

[teor2345]

Since we're still working on the initial implementation, I think this might be a wontfix for now?

[teor2345]

Usually this doesn't actually make a difference to the compiler. Loops and iterators optimise roughly the same.

If we find a hot method (#39), we can try refactorings there.