Align to Scala 2.12 Future API

[danieldietrich]

Viktor Klang created great blog posts on the changes of Future in Scala 2.12:

• Missing canonical combinators: flatten (see Reincarnate flatten() as static method #241) - we use flatMap(t -> t) instead
• Missing canonical combinators: zipWith
• Missing canonical combinators: transform
• Missing canonical combinators: transformWith
• Missing canonical combinators: Deprecations: onSuccess and onFailure - in Vavr these methods return the Future instance, so they can be chained. In our case forEach is not a total version of onSuccess. We keep all methods.
• Missing utilities: unit & never
• Deprecations and Refactors We have refactors, see Removed Promise in favor of Future.join(). Made ForkJoinPool.commonPool() default ExecutorService. #2093, especially we removed Promise.
• Goodbye, sun.misc.Unsafe - this does not apply to Vavr
• Feature: Improved Promise Linking (I promise!) - _Currently out of scope for us. I think the risk in Java isn't that high for OOME because there is no tailrec. _

We should add additional combinators and see if the other blog posts (6-9) are relevant to Javaslang's Future.

See also #1529.

[l0rinc]

FYI: http://www.scala-lang.org/news/2.12.0

[He-Pin]

Alias to Scala 2.13

[danieldietrich]

Our Future implementation will be dropped. There will be no Vavr 1.0 module vavr-concurrent.

Reason: Implementing our own concurrency library is too error-prone, concurrency is a hard problem. There are many(!) alternatives, first of all Java's native CompletableFuture