Faster ascii encode

Before:
```console
$ python3.4 -m timeit -n 10000 -s 'import ujson; x = ["a"*10]*100' 'ujson.dumps(x)'
10000 loops, best of 3: 15.8 usec per loop
```

After:
```console
$ python -m timeit -n 10000 -s 'import ujson; x = ["a"*10]*100' 'ujson.dumps(x)'
10000 loops, best of 3: 7.14 usec per loop
```

Other unicode objects creates utf-8 cache in PyUnicode_AsUTF8AndSize().
It consume extra memory.