pgsql: Refactor the index AM API slightly: move currentItemData and

Log Message:
-----------
Refactor the index AM API slightly: move currentItemData and
currentMarkData from IndexScanDesc to the opaque structs for the
AMs that need this information (currently gist and hash).

Patch from Heikki Linnakangas, fixes by Neil Conway.

Modified Files:
--------------
pgsql/src/backend/access/gist:
gistget.c (r1.63 -> r1.64)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistget.c.diff?r1=1.63&r2=1.64)
gistscan.c (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistscan.c.diff?r1=1.66&r2=1.67)
pgsql/src/backend/access/hash:
hash.c (r1.92 -> r1.93)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hash.c.diff?r1=1.92&r2=1.93)
hashsearch.c (r1.46 -> r1.47)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashsearch.c.diff?r1=1.46&r2=1.47)
pgsql/src/backend/access/index:
genam.c (r1.60 -> r1.61)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/index/genam.c.diff?r1=1.60&r2=1.61)
pgsql/src/include/access:
gist_private.h (r1.25 -> r1.26)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gist_private.h.diff?r1=1.25&r2=1.26)
hash.h (r1.74 -> r1.75)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/hash.h.diff?r1=1.74&r2=1.75)
nbtree.h (r1.108 -> r1.109)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/nbtree.h.diff?r1=1.108&r2=1.109)
relscan.h (r1.51 -> r1.52)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/relscan.h.diff?r1=1.51&r2=1.52)