[SPARK-5038] Add explicit return type for implicit functions.

As we learned in #3580, not explicitly typing implicit functions can lead to compiler bugs and potentially unexpected runtime behavior.

This is a follow up PR for rest of Spark (outside Spark SQL). The original PR for Spark SQL can be found at https://github.com/apache/spark/pull/3859

Author: Reynold Xin <rxin@databricks.com>

Closes #3860 from rxin/implicit and squashes the following commits:

73702f9 [Reynold Xin] [SPARK-5038] Add explicit return type for implicit functions.