Various updates

Merged Martin Reinecke requested to merge tweak_twiddles into master

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?

Edited by Martin Reinecke

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