[SPARK-5038][SQL] Add explicit return type for implicit functions in Spark SQL

As we learned in https://github.com/apache/spark/pull/3580, not explicitly typing implicit functions can lead to compiler bugs and potentially unexpected runtime behavior.

Author: Reynold Xin <rxin@databricks.com>

Closes #3859 from rxin/sql-implicits and squashes the following commits:

30c2c24 [Reynold Xin] [SPARK-5038] Add explicit return type for implicit functions in Spark SQL.