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`
  -  `object never extends Future[Nothing]` in `object Future`
  -  `def defaultBlockContext: BlockContext` in `object BlockContext`
  -  `def toString: String` on stdlib implementations of `Future`

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
  - the `scala.concurrent.impl.DefaultPromise` has been reimplemented to not use `sun.misc.Unsafe`

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)
  - All classes in scala.concurrent.forkjoin. Scala 2.12 will be Java 8+ and as such the jsr166e
    should be used as included in java.util.concurrent.

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
  - Optimizes `object Future.successful` and `object Future.failed` are now separate implementations,
    to optimize for the result, avoiding doing work for the "other branch".
  - Optimizes `compressedRoot()` by avoiding double-calls to volatile get.

Documentation:
  - Almost all methods on `Future` and `Promise` have been revisited and had their ScalaDoc updated

Tests:
  - Yes
SI-8849 Makes `ExecutionContext.Implicits.global` ambiguous

There was an unfortunate side-effect from having `Implicits.global` be
of type `ExecutionContextExecutor`; it is more specific than `ExecutionContext`,
as such it would be picked over other `ExecutionContexts` in the implicit scope.
Merge pull request #4042 from viktorklang/wip-sip14-updates-√

SIP-14 improvements