Use FFT algorithm for faster polynomial multiplication

[jonasvanderschaaf]

This pull request will contain an implementation of the Cooley-Tukey Fast Fourier Transform algorithm, and use it to speed up polynomial multiplication for high degree polynomials. More information can be found in issue #45.

This pull request will contain the following features:

• [x] An implementation of the Fast Fourier Transform
• [x] A modification to the polynomial multiplication code to use this algorithm for high degree polynomial multiplication
• [x] Tests covering most vital functions to ensure proper functioning of multiplication
• [ ] Benchmarking and optimizing the algorithm