Format y2015d15 and add check to prevent overriding days...

src/main.rs

@@ -47,6 +47,9 @@ fn prepare_year(year: &str) -> Result<(), std::io::Error> {
 
 fn prepare_day(year: &str, day: &str) -> Result<(), std::io::Error> {
     let root = env!("CARGO_MANIFEST_DIR");
+    if Path::new(&format!("{root}/y{year}/src/bin/d{day}.rs")).exists() {
+        panic!("Day y{year}d{day} already exists")
+    }
     let bin = fs::read_to_string(format!("{root}/template/bin/d.rs.tmpl"))?
         .replace("{{YEAR}}", year)
         .replace("{{DAY}}", day);