aoc: stop printing inside run functions

This will make benching implementations easier

aoc2019/src/day03.rs

@@ -1,6 +1,7 @@
 use std::collections::HashMap;
 use std::collections::HashSet;
 use std::error::Error;
+use std::fmt::Write;
 use std::str::FromStr;
 
 use aoc::err;
@@ -8,11 +9,13 @@ use aoc::Result;
 
 const INPUT: &str = include_str!("../input/day03.txt");
 
-pub fn run() -> Result<()> {
-    println!("part 1: {}", part1(INPUT)?);
-    println!("part 2: {}", part2(INPUT)?);
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
 
-    Ok(())
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
+
+    Ok(res)
 }
 
 enum Move {