[SPARK-2086] Improve output of toDebugString to make shuffle boundaries more clear

Changes RDD.toDebugString() to show hierarchy and shuffle transformations more clearly

New output:

```
(3) FlatMappedValuesRDD[325] at apply at Transformer.scala:22
 |  MappedValuesRDD[324] at apply at Transformer.scala:22
 |  CoGroupedRDD[323] at apply at Transformer.scala:22
 +-(5) MappedRDD[320] at apply at Transformer.scala:22
 |  |  MappedRDD[319] at apply at Transformer.scala:22
 |  |  MappedValuesRDD[318] at apply at Transformer.scala:22
 |  |  MapPartitionsRDD[317] at apply at Transformer.scala:22
 |  |  ShuffledRDD[316] at apply at Transformer.scala:22
 |  +-(10) MappedRDD[315] at apply at Transformer.scala:22
 |     |   ParallelCollectionRDD[314] at apply at Transformer.scala:22
 +-(100) MappedRDD[322] at apply at Transformer.scala:22
     |   ParallelCollectionRDD[321] at apply at Transformer.scala:22
```
It might be good to make all these inner functions private to avoid having to deal with binary checker changes in the future.