SIP-14 improvements

 Add missing canonical combinators:

  - `def transform[S](f: Try[T] => Try[S])(implicit executor: ExecutionContext): Future[S]`
  - `def transformWith[S](f: Try[T] => Future[S])(implicit executor: ExecutionContext): Future[S]`
  - `def flatten[S](implicit ev: T <:< Future[S]): Future[S]`
  - `def zipWith[U, R](that: Future[U])(f: (T, U) => R)(implicit executor: ExecutionContext): Future[R]`

Add missing utilities:
  -  `val unit: Future[Unit]` in `object Future`
  -  `val never: Future[Nothing]` in `object Future`
  -  `def defaultBlockContext: BlockContext` in `object BlockContext`

Refactors:
  - the `scala.concurrent.Future` trait to not explicit create any `Promises`,
    so that implementations can control implementation type,
    this is mainly facilitated through adding of the `transform` and `transformWith` methods.
  - the implementation of `ExecutionContextImpl` has been cleaned up

Securing:
  - Add a self-check in `completeWith` and `tryCompleteWith` to avoid cycles in trait Promise
  - Capping the maximum number of threads for the global `ExecutionContext` to the max parallelism
  - Implementing (almost) all `Future` combinators on `transformWith` and `transform` means
     that `DefaultPromise` linking works on both `(flat)map` and `recover(With)`
  - Nested `blocking {}` should not spawn extra threads beyond the first.

Removes:
  - the private `internalExecutor` method in favor of an import in trait `Future`
  - the private `internalExecutor` method in favor of an import in trait `Promise`
  - the `AtomicReferenceFieldUpdater` in `AbstractPromise` since we're using `Unsafe`
  - `scala.concurrent.impl.Future` is no longer needed

Deprecates:
  - `Future.onSuccess` - discourage the use of callbacks
                         (and is also redundant considering `foreach` and `onComplete`)
  - `Future.onFailure` - discourage the use of callbacks
                       (and is also redundant considering `onComplete` and `failed.foreach`)
  - `ExecutionContext.prepare` - it was ill specced and it is too easy to forget to call it
                     (or even know when to call it or call it more times than needed)

Reimplements:
  - `failed` - in terms of `transform`
  - `map` - in terms of `transform`
  - `flatMap` - in terms of `transformWith`
  - `recover` - in terms of `transform`
  - `recoverWith` - in terms of `transformWith`
  - `zip` - in terms of `flatMap` + `map`
  - `fallbackTo` - in terms of `recoverWith` + `recoverWith`
  - `andThen` - in terms of `transform`

Miscellaneous:
  - Giving the threads of `ExecutionContext.global` sensible names

Documentation:
  - Almost all methods on `Future` and `Promise` have been revisited and had their ScalaDoc updated

Tests:
  - Yes
Just throwing this out there because I'm a fan of these kinds of constants, but have you looked at twitter util's [ConstFuture](https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Future.scala#L1101-L1141)? It's what they implement their  [constants](https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Future.scala#L19-L22) in terms of. `Nothing` probably covers all cases but I wanted to link to this here in case it was relevant. I love the choice of name for this btw :)