Deck Implementation

Implemented a Deck type that has a new function that returns an
unshuffled deck. Also wrote an integration test for the new function to
make sure that it works.

Reviewed-on: #11

tests/deck_tests.rs

@@ -0,0 +1,18 @@
+use blackjack::Deck;
+
+#[test]
+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);
+
+    for _ in 0..deck.deck_size() {
+        let next_card = deck.get_next_card();
+        let card_pos = next_card.suit * 13 + next_card.value;
+        num_card[card_pos as usize] += 1;
+    }
+
+    for card in 0..num_card.len() {
+        assert_eq!(num_card[card], num_decks);
+    }
+}