Add support for serializing newtype structs

This enables formats like JSON to serialize newtype wrapper
structs without the normal object wrapper. Consider types like:

```rust
struct Point {
    x: u32,
    y: u32,
}

stuct MyPoint(Point);
```

Before this patch, `MyPoint(1,2)` would get serialized as
`[{"x":1,"y":2}]`, but now it gets serialized as `{"x":1,"y"2}`.

serde_json/src/de.rs

@@ -453,6 +453,15 @@ impl<Iter> de::Deserializer for Deserializer<Iter>
         }
     }
 
+    #[inline]
+    fn visit_newtype_struct<V>(&mut self,
+                               _name: &str,
+                               mut visitor: V) -> Result<V::Value, Error>
+        where V: de::Visitor,
+    {
+        visitor.visit_newtype_struct(self)
+    }
+
     #[inline]
     fn visit_enum<V>(&mut self,
                      _name: &str,