pgsql: Extend the MinimalTuple concept to tuplesort.c, thereby reducing

Log Message:
-----------
Extend the MinimalTuple concept to tuplesort.c, thereby reducing the
per-tuple space overhead for sorts in memory. I chose to replace the
previous patch that tried to write out the bare minimum amount of data
when sorting on disk; instead, just dump the MinimalTuples as-is. This
wastes 3 to 10 bytes per tuple depending on architecture and null-bitmap
length, but the simplification in the writetup/readtup routines seems
worth it.

Modified Files:
--------------
pgsql/src/backend/access/nbtree:
nbtsort.c (r1.101 -> r1.102)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsort.c.diff?r1=1.101&r2=1.102)
pgsql/src/backend/executor:
nodeSort.c (r1.56 -> r1.57)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSort.c.diff?r1=1.56&r2=1.57)
pgsql/src/backend/utils/sort:
tuplesort.c (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c.diff?r1=1.66&r2=1.67)
pgsql/src/include/utils:
tuplesort.h (r1.20 -> r1.21)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/tuplesort.h.diff?r1=1.20&r2=1.21)