Function collect_bool_word 

pub fn collect_bool_word<F>(len: usize, f: F) -> u64
where
    F: FnMut(usize) -> bool,

Packs up to 64 boolean values into a little-endian u64 word.

This is collect_bool_words for a single word: a full 64-bit word is materialized as a [bool; 64] and packed with the baseline SIMD byte→bit instruction of the target; shorter lengths fall back to the bit-at-a-time collect_bool_word_scalar loop.