Remove charAt() calls.

No ticket. Remove all `charAt()` calls in favor of `[]` notation (as long as all modern browsers support it) for consistency and size.
Not losing anything in speed: http://jsperf.com/charat-vs-index/2
```
   raw     gz Compared to master @ 8791920183f9f13a7be1d513652b9c96d29648c7    
   -21    -13 dist/jquery.js                                                   
   -21    -18 dist/jquery.min.js   
```