Simplified JSON string handling
Provides a slight boost to performance as well

```sh
       before           after         ratio
     [52f5fdf1]       [325833e0]
     <master>         <faster-json-str>
-         112±1ms        101±0.7ms     0.90  io.json.ToJSON.time_to_json('index', 'df')

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE INCREASED.
```