[SPARK-9301][SQL] Add collect_set and collect_list aggregate functions

For now they are thin wrappers around the corresponding Hive UDAFs.

One limitation with these in Hive 0.13.0 is they only support aggregating primitive types.

I chose snake_case here instead of camelCase because it seems to be used in the majority of the multi-word fns.

Do we also want to add these to `functions.py`?

This approach was recommended here: https://github.com/apache/spark/pull/8592#issuecomment-154247089



marmbrus rxin

Author: Nick Buroojy <nick.buroojy@civitaslearning.com>

Closes #9526 from nburoojy/nick/udaf-alias.

(cherry picked from commit a6ee4f98)
Signed-off-by: Michael Armbrust <michael@databricks.com>