Use Future.successful for std Future's point

Previously Future.apply was used, which uses an implicit
ExecutionContext and has the side effect of beginning the execution of
the Future in a separate thread. This side effect is probably unexpected
from a seemingly-benign point method, and it has quite a bit of
performance overhead.

At first glance, it might appear that a benefit of Future.apply over
Future.successful is that apply is call-by-name while successful is
strict. However, apply is only call-by-name so it can evaluate the
argument in a separate thread. It still (nearly) immediately evaluates
the argument, since standard lib futures immediately begin execution as
opposed to using a run method.

Example of current behavior:
```
scala> Applicative[Future].point(println("hello"))
hello
res9: scala.concurrent.Future[Unit] = scala.concurrent.impl.Promise$DefaultPromise@1d2f62b6

scala> Applicative[Future].point({Thread.sleep(400); println("hi")})
res10: scala.concurrent.Future[Unit] = scala.concurrent.impl.Promise$DefaultPromise@10032b3e

scala> hi
```

While we will still have immediate execution with Future.successful, at
least we will avoid some overhead and won't execute a side-effect as
part of a point call.

I believe this is a candidate for back-porting to 7.1.x.