Investigate bitwise steps

[scottbedard]

There is a possible performance opportunity here, but it needs further investigation.

Instead of FEN index, Position bits could be arranged in the following order,

[
    null,  null,  null,  null,  null,  f11,  g10,   h9,    i8,    k7,    l6,
    null,  null,  null,  null,  e10,   f10,  g9,    h8,    i7,    k6,    l5,
    null,  null,  null,  d9,    e9,    f9,   g8,    h7,    i6,    k5,    l4,
    null,  null,  c8,    d8,    e8,    f8,   g7,    h6,    i5,    k4,    l3,
    null,  b7,    c7,    d7,    e7,    f7,   g6,    h5,    i4,    k3,    l2,
    a6,    b6,    c6,    d6,    e6,    f6,   g5,    h4,    i3,    k2,    l1,
    a5,    b5,    c5,    d5,    e5,    f5,   g4,    h3,    i2,    k1,    null,
    a4,    b4,    c4,    d4,    e4,    f4,   g3,    h2,    i1,    null,  null,
    a3,    b3,    c3,    d3,    e3,    f3,   g2,    h1,    null,  null,  null,
    a2,    b2,    c2,    d2,    e2,    f2,   g1,    null,  null,  null,  null,
    a1,    b1,    c1,    d1,    e1,    f1,   null,  null,  null,  null,  null,
]

And use bitwise shifts to step to new positions

pub fn step(&self, direction: u8) -> Option<Self> {
    let to = match direction {
        0 => self >> 11,
        1 => self >> 21,
        2 => self >> 10,
        3 => self >> 9,
        4 => self << 1,
        5 => seef << 12,
        6 => self << 11,
        7 => self << 21,
        8 => self << 10,
        9 => self << 9,
        10 => self >> 1,
        11 => self >> 12,
        _ => panic!("..."),
    };
    
    match self.get_neighbors() & to {
        0 => None,
        _ => Some(Self::from_bitmask(to),
    }
}

On initial investigation, a gigantic match statement seems fastest, but I'm not ready to let this idea go, there may be something here.