Function prefix_sum

fn prefix_sum(u: usize) -> usize

Compute the prefix sum of each byte within a usize. Let a..h denote the bytes from most significant to least significant and Σx..y denote the sum from x to y inclusive.

    s               |   a   |   b   |   c   |   d   |   e   |   f   |   g   |   h   |
    s += (s >> 8)   |   a   | Σa..b | Σb..c | Σc..d | Σd..e | Σe..f | Σf..g | Σg..h |
    s += (s >> 16)  |   a   | Σa..b | Σa..c | Σa..d | Σb..e | Σc..f | Σd..g | Σe..h |
    s += (s >> 32)  |   a   | Σa..b | Σa..c | Σa..d | Σa..e | Σa..f | Σa..g | Σa..h |