Deprecate scala converters and point users to Scala library equivalent

[Anund McKague]

https://github.com/scala/scala-java8-compat appears to make Scala converters redundant. We should move to minimizing our overlap with that library and point users in it's direction. Thanks to @jbgi for pointing this out.

[Jean-Baptiste Giraudeau]

Maybe there is still a need for fugue Option <-> scala Option, fugue Either <-> scala Either and fugue Pair <-> scala Tuple2 converters.

[Anund McKague]

timing out