use u64 for occupancy

[van-sprundel]

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📊 Benchmark Results (Instruction Counts)

Comparing add-arrayvec-and-bitmap (PR) vs master (base)

Using iai-callgrind: Counts instructions via Cachegrind - deterministic and not affected by CI noise.

moves::single move: 5953|6330 ((-5.95577%))
moves::opponent moves: 13901|14000 ((-0.70714%))
moves::all moves: 28769|29146 ((-1.29349%))
moves::all legal moves: 435193|426306 ((+2.08465%))
piece moves::pawn: 6113|6490 ((-5.80894%))
piece moves::king: 5894|6271 ((-6.01180%))
piece moves::knight: 6175|6552 ((-5.75397%))
piece moves::queen: 6145|6522 ((-5.78044%))

_{📏 Instruction counts via Cachegrind. Lower is better. Results are deterministic.}