pgsql: Do execGrouping.c via expression eval machinery, take two.

Do execGrouping.c via expression eval machinery, take two.

This has a performance benefit on own, although not hugely so. The
primary benefit is that it will allow for to JIT tuple deforming and
comparator invocations.

Large parts of this were previously committed (773aec7aa), but the
commit contained an omission around cross-type comparisons and was
thus reverted.

Author: Andres Freund
Discussion: https://postgr.es/m/20171129080934.amqqkke2zjtekd4t@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/bf6c614a2f2c58312b3be34a47e7fb7362e07bcb

Modified Files
--------------
src/backend/executor/execExpr.c           | 143 +++++++++++++++++
src/backend/executor/execExprInterp.c     |  29 ++++
src/backend/executor/execGrouping.c       | 249 +++++++-----------------------
src/backend/executor/nodeAgg.c            | 145 ++++++++++-------
src/backend/executor/nodeGroup.c          |  24 +--
src/backend/executor/nodeRecursiveunion.c |  11 +-
src/backend/executor/nodeSetOp.c          |  54 +++----
src/backend/executor/nodeSubplan.c        | 110 +++++++++++--
src/backend/executor/nodeUnique.c         |  31 ++--
src/backend/executor/nodeWindowAgg.c      |  38 +++--
src/backend/utils/adt/orderedsetaggs.c    |  56 +++----
src/include/executor/execExpr.h           |   1 +
src/include/executor/executor.h           |  34 ++--
src/include/executor/nodeAgg.h            |  14 +-
src/include/nodes/execnodes.h             |  27 ++--
15 files changed, 566 insertions(+), 400 deletions(-)