SPARK-2813:  [SQL] Implement SQRT() directly in Spark SQL

This PR adds a native implementation for SQL SQRT() and thus avoids delegating this function to Hive.

Author: William Benton <willb@redhat.com>

Closes #1750 from willb/spark-2813 and squashes the following commits:

22c8a79 [William Benton] Fixed missed newline from rebase
d673861 [William Benton] Added string coercions for SQRT and associated test case
e125df4 [William Benton] Added ExpressionEvaluationSuite test cases for SQRT
7b84bcd [William Benton] SQL SQRT now properly returns NULL for NULL inputs
8256971 [William Benton] added SQRT test to SqlQuerySuite
504d2e5 [William Benton] Added native SQRT implementation