Support the Iterator API from the original Jsoniter

[lexspoon]

I'm experimenting with Jsoniter-scala, and it has gone well in general.

One thing that can be difficult, though, is custom deserializers. With the original Jsoniter, there is a very nice Iterator API. The web site goes through some of the advantages, and I am finding them to be true in practice. Writing a custom deserializer with the JsonReader interface is tedious and error prone.

It may be possible to share code with the upstream project that inspired this one, if that code is refactored a little bit with some abstract interfaces. Or, maybe just import the JsonIter class into the jsoniter-scala project, and modify it to work on top of a JsonReader while keeping the same public methods. I don't know which way is best, but from a user point of view, having a way to use the iterator API would be really great.