Update edition to 2021

Cargo.toml

@@ -1,6 +1,7 @@
 [package]
 name = "ico"
 version = "0.2.0"
+edition = "2021"
 authors = ["Matthew D. Steele <mdsteele@alum.mit.edu>"]
 description = "A library for encoding/decoding ICO image files"
 repository = "https://github.com/mdsteele/rust-ico"

examples/icotool.rs

@@ -1,6 +1,3 @@
-extern crate clap;
-extern crate ico;
-
 use clap::{App, Arg, SubCommand};
 use std::fs;
 use std::path::PathBuf;

src/lib.rs

@@ -56,13 +56,9 @@
 
 #![warn(missing_docs)]
 
-extern crate byteorder;
-extern crate png;
-
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
 use std::collections::{BTreeSet, HashMap};
 use std::io::{self, Read, Seek, SeekFrom, Write};
-use std::{u16, u8};
 
 // ========================================================================= //