Codegen SUM(decimal)

On a simple local benchmark that sums a decimal column, this improves
query time by ~4x.

Change-Id: I4968a31c66f94ac55b1d3a6e51707f72b81b662a
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4341
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins