[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
given how large this file is, it might be worth it to create a cg package (or not) and break the file into multiple files.