Sukaku Explainer v1.2.1.6

Speed improvements over previous v1.2.1.5 and minor stabilization.
Nested chains are processed in parallel.
Hardest vanilla sudoku are processed 2 times faster on 4 core machine.
Easy and moderate puzzles are processed ~10% faster.
Introduced are new experimental solving techniques beyond known hardest vanilla puzzles. Finds eliminations on hardest Pencilmark-only puzzles but still can't solve them.
Rating of all vanilla puzzles isn't affected.