Optimize attackers_to()

[mstembera]

https://tests.stockfishchess.org/tests/view/6782decb6ddf09c0b4b6e1b0
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 105920 W: 27571 L: 27181 D: 51168
Ptnml(0-2): 284, 10808, 30403, 11164, 301

• If we only need to know if attackers exist we can skip some calculations.
• Also calculating slider/magic attackers first is better because the double lookup is slow due to memory latency.
• I also included a couple of very minor cleanups in search that probably don't warrant their own PR but I can open separately if that's better.

No functional change
bench: 999324

[github-actions]

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(execution 12728284853 / attempt 1)