Provide deprecated compatibility stubs for the types and static members,
which forward as follows:

```
scala.concurrent.forkjoin.ForkJoinPool         => java.util.concurrent.ForkJoinPool
scala.concurrent.forkjoin.ForkJoinTask         => java.util.concurrent.ForkJoinTask
scala.concurrent.forkjoin.ForkJoinWorkerThread => java.util.concurrent.ForkJoinWorkerThread
scala.concurrent.forkjoin.LinkedTransferQueue  => java.util.concurrent.LinkedTransferQueue
scala.concurrent.forkjoin.RecursiveAction      => java.util.concurrent.RecursiveAction
scala.concurrent.forkjoin.RecursiveTask        => java.util.concurrent.RecursiveTask
scala.concurrent.forkjoin.ThreadLocalRandom    => java.util.concurrent.ThreadLocalRandom
```

To prepare for Java 9, the Scala library does not itself use `sun.misc.Unsafe`.
However, for now, it provide a convenience accessor for it
via `scala.concurrent.util.Unsafe`. This (deprecated) class will
be removed as soon as the eco-system drops its use
(akka-actor, I'm looking at you).

Use j.u.c.Forkjoin directly in active and disabled tests

Remove bitrotted benchmarks code

I was going to update these to use `java.util.concurrent.ForkJoin`
directly, instead of our deprecated stubs.

But most of them don't compile anymore (e.g. scala.testing.Benchmark
has been removed, ClassTag imports missing).

While I'm all for benchmarks, we should have large swathes of code
checked in that isn't at compiled and run automatically.

I'm happy to help someone resurrect these in a suitable form.