Integration with scala-json AST

[mdedetrich]

After the results of SLIP-28/scala contributors, the ScalaJSON AST has finally been put into the scala platform (see https://github.com/mdedetrich/scalajson for more details). Currently its a 1.0.0-M1 release however a full release is expected in a few days if there are no major issues).

This ticket is to describe integration with Jawn, and how it would possibly work. A really ideal situation would be to integrate the scalajson.ast.unsafe.JValue into the core Jawn parser (and I would also be interested to see the results in regards to parsing). Doing so would also automatically expose the standard safe AST to any users and would provide "automatic" interopt once the other frameworks/json libraries adopt scalaJSON.

If this doesn't work out, it can always be added as either a module (least ideal situation) or a first class type that jawn exposes. Let me know if there are any ways that I can help!