prefer json4s-ast for json4s use in jawn

If all the facades are doing is emitting support-specific types, it may be to depend on json4s-ast rather tan json4s-native. the native package includes parsing logic not used here an some other stuff in core which just [alias](https://github.com/json4s/json4s/blob/master/core/src/main/scala/org/json4s/package.scala#L23-L41) the ast at the json4s package level. This should also make the jawn facade for json4s a better fit if you depend on one of the other json4s-* flavored parsers: json4s-jackson, ect.