Optimize attackers_to()

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.

closes https://github.com/official-stockfish/Stockfish/pull/5762

No functional change

src/position.h

@@ -126,6 +126,7 @@ class Position {
     // Attacks to/from a given square
     Bitboard attackers_to(Square s) const;
     Bitboard attackers_to(Square s, Bitboard occupied) const;
+    bool     attackers_to_exist(Square s, Bitboard occupied, Color c) const;
     void     update_slider_blockers(Color c) const;
     template<PieceType Pt>
     Bitboard attacks_by(Color c) const;