WIP: String streaming (w/ separate JSON string reader cls)

[smheidrich]

2nd attempt at something that looks more like how it's done in json-stream, i.e. with a separate JSON string reader class.

Equivalent of daggaz/json-stream#47 on json-stream side.

Supersedes #84 and #89.

ToDos:

• Change string handling in RustTokenizer
• JsonStringReader impl.
  • read
  • __next__
  • readline
  • completed or sth.
    • Is that needed? I think read returning an empty string should be indication that it's done enough.
  • Make subclass of *IOBase?
    • Isn't easily possible: Docs should mention that it's not possible to derive Rust classes from Python ones PyO3/pyo3#3415 I guess this could be worked around by having a wrapper class on the Python side but I'm not sure it's worth it...
  • ... TODO ...
    • Nothing important comes to mind.
• Tests
  • Using json-stream's (with some xfail where not important)
  • Our own (integration tests)

[smheidrich]

@daggaz Just to let you know: I think the most important features here are finally ready (namely, returning strings as objects that can be read() from and iterated over, if requested via strings_as_files=True) and I'm not sure the other ones (full IOBase interface implementation, exactly aligning exceptions with the Python tokenizer, etc.) are required at this point.

So I'm prepared to merge this when you are 👍