[WIP][SPARK-2054][SQL] Code Generation for Expression Evaluation

This PR drops the aggregation code generation, which will be added back in a follow-up PR.  The following remains to be done:
 - [ ] Remove remaining spurious changes.
 - [ ] Make configurable with #956
should we consider moving code gen for each expression into the expression itself? just like what we did with eval.

Just throwing an idea out there. I can see benefits on both approaches.