Integrating with ScalaJSON AST

The efforts from both SLIP-28 and the scalaplatform has finally been materialized, ScalaJSON AST is now released on the scalaplatfrom with its first milestone release (see https://github.com/mdedetrich/scalajson for more information). Its expected that a final release will be made in a few days pending that there aren't many major issues.

This ticket is meant to deal with any potential migration issues, also note that there already exists an integration at https://github.com/mdedetrich/playframework/tree/scalaJsonAstIntegration (although this is with an older version of Play 2.5).

There are different ways to integrate the AST, one would be with (mainly) source compatibility which is the reason behind https://github.com/mdedetrich/playframework/blob/b0934ce452d82312f37cd62af28524c6840a4c45/framework/src/play-json/src/main/scala/play/api/libs/json/package.scala#L47-L58. Another alternative would be to just deal with the fact that JsValue is now just JValue (same goes for all other json types). In any case I suspect that this is going to be a big breaking change that will require a major release (play-json version 2.7.x to coincide with play 2.7 release).

I can also help with the integration once I know what the goals are

Compare the existing scalaJsonAstIntegration branch with the current play-json code, starting at framework/src/play-json/src/main/scala/play/api/libs/json/package.scala and its compatibility definitions. Review the ScalaJSON AST release and the linked integration to identify migration issues; done criteria depend on agreeing whether source compatibility or direct JsValue-to-JValue migration is required.