fixedIntFromSignedDoubleLimb

fn fixedIntFromSignedDoubleLimb(A: SignedDoubleLimb, storage: []Limb) Mutable {
    assert(storage.len >= 2);

    const A_is_positive = A >= 0;
    const Au = @as(DoubleLimb, @intCast(if (A < 0) -A else A));
    storage[0] = @as(Limb, @truncate(Au));
    storage[1] = @as(Limb, @truncate(Au >> limb_bits));
    return .{
        .limbs = storage[0..2],
        .positive = A_is_positive,
        .len = 2,
    };
}