day5: performance: don't iterate to position just do arithemtic

5/src/main.rs

@@ -163,10 +163,7 @@ impl From<&str> for AlmanacMapping {
 impl AlmanacMapping {
     fn lookup(&self, key: u64) -> Option<u64> {
         if self.source_range.contains(&key) {
-            match self.source_range.clone().position(|x| x == key) {
+            Some(self.dest_range.start + (key - self.source_range.start))
-                Some(i) => self.dest_range.clone().nth(i),
-                None => None,
-            }
         } else {
             None
         }