JSONReader, JSONWriter, Reader and Writer?

[jdolan]

If the StringReader proves useful, it'd make sense to introduce counterparts for other common stream operations, namely:

• Consider adding a Reader class that operates on Data just as StringReader operates on String.
• This class would be more straightforward, since it doesn't have to deal with Unicode directly.
• JSONReader and JSONWriter are currently private structs that operate on a Data instance. Might be nice to hoist these up to classes.
• It would be ideal if some inheritance model existed between these classes:
• Reader
• StringReader
• JSONReader -or-
  * JSONReader

This approach would diverge from Foundation's NSInputStream model, and that should be weighed before we go too far down this path also.