whisper: use global cache for sin/cos vals and Hann window

[iboB]

...also rename Hanning to Hann (named after Julius von Hann as per Wikipedia)

• Hann window
  • Saves a computation for each run.
• sin/cos tables:
  • slight optimization as it doesn't check and synchronize a static var
  • fix a (mostly benign) race condition where if two threads start computing them at the same time, both will end-up doing so

This at the cost of a very slightly slower boot time.