SPARK-2028: Expose mapPartitionsWithInputSplit in HadoopRDD

This allows users to gain access to the InputSplit which backs each partition.

An alternative solution would have been to have a .withInputSplit() method which returns a new RDD[(InputSplit, (K, V))], but this is confusing because you could not cache this RDD or shuffle it, as InputSplit is not inherently serializable.
Add `@DeveloperApi` on these, and also on mapPartitionsWithInputSplit in general (since it requires casting stuff to HadoopRDD)