Optimize polynomial multiplication

[emmorais]

Current multiplication takes 60 ms, while NTL and FLINT take roughly 2 ms for the same task.

• [ ] Compare algorithms chosen for similar parameters on libraries like NTL and FLINT.
• [ ] Implement schoolbook version of the algorithm.
• [ ] Implement optimizations (long-hanging fruits).
• [ ] Benchmark again.
• [ ] Documentation.

[emmorais]

Notion page showing the Karatsuba method (adapted for cyclotomic rings): https://www.notion.so/inversed/Karatsuba-method-for-polynomials-566b1d9a6aa64184bc9c92d1da9bcb37?pvs=4