Author: Reynold Xin <rxin@databricks.com>

Closes #6536 from rxin/structural-type-checker and squashes the following commits:

f833151 [Reynold Xin] Fixed compilation.
633f9a1 [Reynold Xin] Fixed typo.
d1fa804 [Reynold Xin] [SPARK-7979] Enforce structural type checker.