pgsql: Assorted cosmetic cleanup of run-time-partition-pruning code.

Assorted cosmetic cleanup of run-time-partition-pruning code.

Use "subplan" rather than "subnode" to refer to the child plans of
a partitioning Append; this seems a bit more specific and hence
clearer.  Improve assorted comments.  No non-cosmetic changes.

David Rowley and Tom Lane

Discussion: https://postgr.es/m/CAFj8pRBjrufA3ocDm8o4LPGNye9Y+pm1b9kCwode4X04CULG3g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/321f648a312bd188b7a5c33a0548a57f45dfbf42

Modified Files
--------------
src/backend/executor/execPartition.c | 163 +++++++++++++++++------------------
src/backend/nodes/copyfuncs.c        |   2 +-
src/backend/nodes/outfuncs.c         |   4 +-
src/backend/nodes/readfuncs.c        |   2 +-
src/backend/partitioning/partprune.c |  68 +++++++--------
src/include/executor/execPartition.h |  48 ++++++-----
src/include/nodes/plannodes.h        |  16 ++--
7 files changed, 147 insertions(+), 156 deletions(-)