Various updates

This MR contains several fixes/improvements:

• warning fixes for clang (hopefully without breaking MSVC again!)
• switching back to the system sin/cos functions for twiddles (less code and no real loss of precision)
• compact storage of twiddles in sincos_2pibyn (only O(sqrt(N)) instead of O(N)).
• use sincos_2pibyn for twiddle calculation in some DCTs/DSTs, where sin/cos were called directly so far
• size reduction in Bluestein plans by making use of symmetries.

@g-peterbell any comments?

bench.py

 import numpy as np
 import pypocketfft
-import pyfftw
 from time import time
 import matplotlib.pyplot as plt
 import math