more efficient handling for smalls strings inside json_object

smalls strings inside json_objects had a high overhead because dynamic
memory allocation was needed for each of them. This also meant that the
pointer needed to be updated. This is now changed so that small strings
can directly be stored inside the json_object. Note that on the regular
64 bit machines a pointer takes 8 bytes. So even without increasing
memory, we could store string up to 7 bytes directly inside the object.
The max size is configurable. I have selected up to 31 bytes (which
means a buffer of 32 including the NUL byte). This brings a 24-bytes
memory overhead, but I consider that still useful because the memory
allocator usually also has quite some overhead (16 bytes) for
dyn alloced memory blocks. In any case, the max buffer size can be
tweaked via #define.

json_object_private.h

@@ -16,6 +16,8 @@
 extern "C" {
 #endif
 
+#define LEN_DIRECT_STRING_DATA 32 /**< how many bytes are directly stored in json_object for strings? */
+
 typedef void (json_object_private_delete_fn)(struct json_object *o);
 
 struct json_object
@@ -32,7 +34,13 @@ struct json_object
     struct lh_table *c_object;
     struct array_list *c_array;
     struct {
-        char *str;
+	union {
+		/* optimize: if we have small strings, we can store them
+		 * directly. This saves considerable CPU cycles AND memory.
+		 */
+		char *ptr;
+		char data[LEN_DIRECT_STRING_DATA];
+	} str;
         int len;
     } c_string;
   } o;