future/fixed: update fixed-point implementation to match latest lib.fixed RFC

[vk2seb]

• Modify Tiliqua codebase to use my latest implementation of the lib.fixed RFC.
  • Goal is to experiment with the ergonomics of lib.fixed and make sure it interacts with the rest of Amaranth and my existing DSP components without unexpected surprises.
• lib.fixed is added out-of-tree under amaranth_future.fixed, so we can still point to upstream Amaranth releases.
  • This includes a large test suite of lib.fixed that is now run against amaranth_future.fixed.
• Improvements along the way:
  • Remove all the signed/unsigned hacks that polluted tiliqua.dsp, now fixed by lib.fixed.
  • Remove Memory instances that were initialized by normal Value when they could be initialized by fixed.Value.
  • Rewrite the dsp.Boxcar implementation to use delay_line, and move it to delay.
  • Implement a DCBlock DSP component and use it in polysyn.
  • Increase RingMAC/MuxMAC multiplier result width, this is used for fraction saving and by DCBlock.
  • Increase accumulator widths in SVF/MatrixMix to reduce noise and DC bias.