Don't zero the array on initialization

Using `mem::uninitialized` is potentially faster than `mem::zeroed`, and
doesn't remove any safety (since both uninitialized memory and zeroed
memory are unsafe to read from in general).

lib.rs

@@ -202,7 +202,7 @@ impl<A: Array> SmallVec<A> {
         unsafe {
             SmallVec {
                 len: 0,
-                data: Inline { array: mem::zeroed() },
+                data: Inline { array: mem::uninitialized() },
             }
         }
     }