clean up project

- modularized code into individual day modules
- moved utilities into utils module
- added unit testing

src/utils.rs

@@ -0,0 +1,13 @@
+use anyhow::Result;
+use std::fs;
+
+pub fn read_i64s(filename: &str) -> Result<Vec<i64>> {
+    Ok(read_file(filename)?
+        .lines()
+        .map(|s| s.parse::<i64>().expect("i64 parse error"))
+        .collect())
+}
+
+pub fn read_file(filename: &str) -> Result<String> {
+    Ok(fs::read_to_string(filename)?)
+}