Function collect_bool_words 

pub fn collect_bool_words<F>(words: &mut [u64], len: usize, f: F)
where
    F: FnMut(usize) -> bool,

Pack len boolean values returned by f into the prefix of words, LSB-first, 64 bits per u64. words must have capacity for at least len.div_ceil(64) entries.

Writes via = (not |=), so the destination need not be zero-initialised.