Move to new slice syntax

This fixes some warnings.

Author: drewcrawford

tests/test.rs

@@ -25,11 +25,11 @@ fn times_two(n: u32) -> u32 {
 
 #[test]
 fn test_basic() {
-    assert_eq!(STRING.as_slice(), "hello");
+    assert_eq!(&STRING[..], "hello");
     assert_eq!(*NUMBER, 6);
     assert!(HASHMAP.get(&1).is_some());
     assert!(HASHMAP.get(&3).is_none());
-    assert_eq!(ARRAY_BOXES.as_slice(), [Box::new(1), Box::new(2), Box::new(3)].as_slice());
+    assert_eq!(&ARRAY_BOXES[..], &[Box::new(1), Box::new(2), Box::new(3)][..]);
 }
 
 #[test]