boilerplate: make generic over Lines type

boilerplate/src/main.rs

@@ -17,11 +17,11 @@ fn main() {
 
 // PROBLEM 1 solution
 
-fn problem1(input: InputIter) -> u64 {
+fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
     0
 }
 
 // PROBLEM 2 solution
-fn problem2(input: InputIter) -> u64 {
+fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
     0
 }