product: Büchi|Büchi=Büchi, CoBüchi&CoBüchi=CoBüchi
Improve the construction of the above constructions, saving colors.
- spot/twaalgos/product.cc: Here.
- spot/twaalgos/product.hh, NEWS: Mention it.
- tests/core/prodchain.test, tests/core/prodor.test, tests/python/_product_weak.ipynb: Adjust.