Deck Shuffling

Implemented a deck shuffling algorithm. It will iterate over the deck of
cards 1000 times and swap the card it is on with another random card to
mix up the deck.

Reviewed-on: #12

tests/deck_tests.rs

@@ -5,6 +5,7 @@ fn check_num_decks() {
     let mut num_card: [u64; 52] = [0; 52];
     let num_decks = 8;
     let mut deck = Deck::new(num_decks as u64);
+    deck.shuffle();
 
     for _ in 0..deck.deck_size() {
         let next_card = deck.get_next_card();