Add radix2 FFT (#1166)

* Add radix2 FFT

Created a dynamic implementation of the radix - 2 Fast Fourier Transform for fast polynomial multiplication.

Reference: https://en.wikipedia.org/wiki/Cooley%E2%80%93Tukey_FFT_algorithm#The_radix-2_DIT_case

* Rename radix2_FFT.py to radix2_fft.py

* Update radix2_fft printing 

Improved the printing method with f.prefix and String.join()

* __str__ method update

* Turned the tests into doctests