[SPARK-7937][SQL] Support comparison on StructType

This brings #6519 up-to-date with master branch.

Closes #6519.

Author: Liang-Chi Hsieh <viirya@appier.com>
Author: Liang-Chi Hsieh <viirya@gmail.com>
Author: Reynold Xin <rxin@databricks.com>

Closes #7877 from rxin/sort-struct and squashes the following commits:

4968231 [Reynold Xin] Minor fixes.
2537813 [Reynold Xin] Merge branch 'compare_named_struct' of github.com:viirya/spark-1 into sort-struct
d2ba8ad [Liang-Chi Hsieh] Remove unused import.
3a3f40e [Liang-Chi Hsieh] Don't need to add compare to InternalRow because we can use RowOrdering.
dae6aad [Liang-Chi Hsieh] Fix nested struct.
d5349c7 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into compare_named_struct
43d4354 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into compare_named_struct
1f66196 [Liang-Chi Hsieh] Reuse RowOrdering and GenerateOrdering.
f8b2e9c [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into compare_named_struct
1187a65 [Liang-Chi Hsieh] Fix scala style.
9d67f68 [Liang-Chi Hsieh] Fix wrongly merging.
8f4d775 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into compare_named_struct
94b27d5 [Liang-Chi Hsieh] Remove test for error on complex type comparison.
2071693 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into compare_named_struct
3c142e4 [Liang-Chi Hsieh] Fix scala style.
cf58dc3 [Liang-Chi Hsieh] Use checkAnswer.
f651b8d [Liang-Chi Hsieh] Remove Either and move orderings to BinaryComparison to reuse it.
b6e1009 [Liang-Chi Hsieh] Fix scala style.
3922b54 [Liang-Chi Hsieh] Support ordering on named_struct.