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

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