Function encode_and_popcount 

pub fn encode_and_popcount<R: Rng + ?Sized>(
    weight_words: &[u64],
    prob: f64,
    length: usize,
    rng: &mut R,
) -> u64

Fused encode+AND+popcount without materializing encoded input words.

Semantically equivalent to:

1. encoded = bernoulli_packed_simd(prob, length, rng)
2. sum(popcount(encoded[word] & weight_words[word]))