Wip sip23 √

@richdougherty 

>    class ExecutionContextWithLocals(map: mutable.Map[String,Any]) extends ExecutionContext {
>      import ExecutionLocals._
>      def execute(r: Runnable) = {
>        ...
>        val prev = threadLocal.get
>        threadLocal.set(map)
>        try r.run() finally threadLocal.set(prev)
>        ...
>      }
>    }

See: https://gist.github.com/viktorklang/8652944

Looks familiar?