Create advent prelude and cleanup lint by using it.

2021/src/day15.rs

@@ -1,15 +1,5 @@
-use std::{
-    collections::{HashMap, HashSet, VecDeque},
-    convert::Infallible,
-    fmt::{Debug, Error, Formatter},
-    num::ParseIntError,
-    ops::{Index, IndexMut},
-    str::FromStr,
-};
-
-use anyhow::Result;
-use aoc_runner_derive::{aoc, aoc_generator};
-use thiserror::Error;
+use advent::prelude::*;
+use aoc_runner_derive::aoc;
 
 struct Image {
     width: usize,