overhaul chain system

This made the code so fast, that the `board` benchmark became completely useless.
So that benchmark was removed.

Overall, we're looking at a futher 20% performance increase in `future_moves`

benches/board.rs

@@ -1,29 +0,0 @@
-use criterion::{criterion_group, criterion_main, Criterion, Throughput};
-use othello::repr::{Board, Piece};
-
-fn fill_board(loops: usize) {
-    let mut board = Board::new();
-    board.place_unchecked((0, 1).into(), Piece::White);
-    board.place_unchecked((0, 2).into(), Piece::Black);
-    board.place_unchecked((0, 3).into(), Piece::Black);
-    board.place_unchecked((0, 4).into(), Piece::Black);
-    for _ in 0..loops {
-        let mut board = board;
-        let _ = board.place((0, 5).into(), Piece::White);
-    }
-}
-
-fn criterion_benchmark(c: &mut Criterion) {
-    let mut group = c.benchmark_group("board");
-
-    const LOOPS: usize = 1000;
-
-    group.throughput(Throughput::Elements(LOOPS as u64));
-    group.bench_function("board_place", |b| {
-        b.iter(|| fill_board(LOOPS));
-    });
-    group.finish();
-}
-
-criterion_group!(benches, criterion_benchmark);
-criterion_main!(benches);