SPARK-2519. Eliminate pattern-matching on Tuple2 in performance-critical...

... aggregation code

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1435 from sryza/sandy-spark-2519 and squashes the following commits:

640706a [Sandy Ryza] SPARK-2519. Eliminate pattern-matching on Tuple2 in performance-critical aggregation code
[SPARK-2518][SQL] Fix foldability of Substring expression.

This is a follow-up of #1428.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1432 from ueshin/issues/SPARK-2518 and squashes the following commits:

37d1ace [Takuya UESHIN] Fix foldability of Substring expression.
[SPARK-2517] Remove some compiler warnings.

Author: Reynold Xin <rxin@apache.org>

Closes #1433 from rxin/compile-warning and squashes the following commits:

8d0b890 [Reynold Xin] Remove some compiler warnings.