Slightly more efficient and compact code.

This version loops the input twice, first to build the claims, and then
to check whether they have been claimed more than once.

2018/src/main.rs

@@ -1,6 +1,7 @@
 extern crate clap;
 extern crate chrono;
 extern crate regex;
+#[macro_use] extern crate itertools;
 use clap::{Arg, App};
 use std::fs;
 use std::io;