oberheim/xpander.cpp: Moved voice board logic into its own device. (#15867)

Code reorganization. No functional changes.

* Created a preliminary device for the xpander voice.
* Created a preliminary device for the xpander voice board.
  * Includes the 6 voices.
* Moved relevant logic from `xpander_state` into the two devices above.
* Switched from floats to doubles.
* Other minor code reorganization.
* Stylistic fixes.

Author: m1macrophage