JIRA: https://issues.apache.org/jira/browse/SPARK-8237

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6934 from viirya/expr_sha2 and squashes the following commits:

35e0bb3 [Liang-Chi Hsieh] For comments.
68b5284 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_sha2
8573aff [Liang-Chi Hsieh] Remove unnecessary Product.
ee61e06 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_sha2
59e41aa [Liang-Chi Hsieh] Add misc function: sha2.