Fix: lock stdin when reading from it buffered

Buffered reading from stdin now needs to lock stdin. This is to solve
rust-lang/rust#14434 which was a race condition on buffered reads from
stdin.

This change is to conform to the new API submitted as part of
rust-lang/rust#19416.

Author: muggenhor

src/main.rs

@@ -12,7 +12,7 @@ mod test;
 
 #[cfg(not(test))]
 fn main() {
-    for e_line in std::io::stdin().lines() {
+    for e_line in std::io::stdin().lock().lines() {
         match e_line {
             Ok(line) => {
                 let mut cur_puzzle = create_puzzle(line.as_slice());