Ability to report back slices of printable characters

The trait currently reports a `char` at a time. While terminal emulators might need to operate on `char`s, other tools like stripping of ANSI escape codes or converting them, need to work on slices
- `&str` -> `char` conversion is expensive, relatively speaking
- doing IO per `char` would be expensive
- allocating a `String` to aggregate the `char`s would also be expensive

To workaround this, `anstream` exposed the state changes from its fork, `anstyle-parse`, to do this.