pgsql: Remove the no-longer-useful BTItem/BTItemData level of structure,

Log Message:
-----------
Remove the no-longer-useful BTItem/BTItemData level of structure, and
just refer to btree index entries as plain IndexTuples, which is what
they have been for a very long time. This is mostly just an exercise
in removing extraneous notation, but it does save a palloc/pfree cycle
per index insertion.

Modified Files:
--------------
pgsql/src/backend/access/nbtree:
nbtinsert.c (r1.131 -> r1.132)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtinsert.c.diff?r1=1.131&r2=1.132)
nbtpage.c (r1.91 -> r1.92)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtpage.c.diff?r1=1.91&r2=1.92)
nbtree.c (r1.135 -> r1.136)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtree.c.diff?r1=1.135&r2=1.136)
nbtsearch.c (r1.102 -> r1.103)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsearch.c.diff?r1=1.102&r2=1.103)
nbtsort.c (r1.97 -> r1.98)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsort.c.diff?r1=1.97&r2=1.98)
nbtutils.c (r1.70 -> r1.71)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtutils.c.diff?r1=1.70&r2=1.71)
nbtxlog.c (r1.24 -> r1.25)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtxlog.c.diff?r1=1.24&r2=1.25)
pgsql/src/include/access:
nbtree.h (r1.90 -> r1.91)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/nbtree.h.diff?r1=1.90&r2=1.91)