Wip sip23 √

It seems to me that it might make sense to port [Twitter's locals](https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Local.scala) to Scala's Future to evaluate the different proposals. I suppose there's a corresponding test suite for locals that we could port as well. Otherwise, it seems difficult to ensure we cover one of the more important use cases. What do people think?