Use u64 to represent solution

Some puzzles have solutions which are too large for `u32`s.

src/template.txt

@@ -1,10 +1,10 @@
 advent_of_code::solution!(%DAY_NUMBER%);
 
-pub fn part_one(input: &str) -> Option<u32> {
+pub fn part_one(input: &str) -> Option<u64> {
     None
 }
 
-pub fn part_two(input: &str) -> Option<u32> {
+pub fn part_two(input: &str) -> Option<u64> {
     None
 }