Function aoc::year2021::day23::expand

fn expand(
    todo: &mut MinHeap<usize, Burrow>,
    seen: &mut FastMap<Burrow, usize>,
    burrow: Burrow,
    energy: usize,
    room_index: usize,
    iter: impl Iterator<Item = usize>,
)

Searches the hallway in either the right or left direction, pushing a new state to the priority queue if it’s possible to place an amphipod there.