pgsql: Move generic slot support functions from heaptuple.c into execTu

Move generic slot support functions from heaptuple.c into execTuples.c.

heaptuple.c was never a particular good fit for slot_getattr(),
slot_getsomeattrs() and slot_getmissingattrs(), but in upcoming
changes slots will be made more abstract (allowing slots that contain
different types of tuples), making it clearly the wrong place.

Note that slot_deform_tuple() remains in it's current place, as it
clearly deals with a HeapTuple. getmissingattrs() also remains, but
it's less clear that that's correct - but execTuples.c wouldn't be the
right place.

Author: Ashutosh Bapat.
Discussion: https://postgr.es/m/20180220224318.gw4oe5jadhpmcdnm@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9d906f1119de893a4ca533c5e7b97207a3aa963b

Modified Files
--------------
src/backend/access/common/heaptuple.c | 188 +--------------------------------
src/backend/executor/execTuples.c | 190 ++++++++++++++++++++++++++++++++++
src/include/access/htup_details.h | 2 +
src/include/executor/tuptable.h | 10 +-
4 files changed, 201 insertions(+), 189 deletions(-)