Make operator<< to return void

This should help preventing misuse.

No functional change.
Queen vs. 3 pieces imbalance

Passed both STC
LLR: 7.32 (-2.94,2.94) [-1.50,4.50]
Total: 98108 W: 18087 L: 17576 D: 62445

And LTC
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 15082 W: 2417 L: 2248 D: 10417

bench: 7717336