Add hstore contrib module.

Per discussion
http://archives.postgresql.org/pgsql-hackers/2006-08/msg01409.php

contrib/hstore/hstore.h

@@ -0,0 +1,49 @@
+#ifndef __HSTORE_H__
+#define __HSTORE_H__
+
+#include "postgres.h"
+#include "funcapi.h"
+#include "access/gist.h"
+#include "access/itup.h"
+#include "utils/elog.h"
+#include "utils/palloc.h"
+#include "utils/builtins.h"
+#include "storage/bufpage.h"
+
+
+typedef struct {
+	uint16	keylen;
+	uint16	vallen;
+	uint32  
+		valisnull:1,
+		pos:31;
+} HEntry; 
+
+
+typedef struct {
+	int4	len;
+	int4	size;
+	char	data[1];
+} HStore;
+
+#define HSHRDSIZE	(2*sizeof(int4))
+#define CALCDATASIZE(x, lenstr) ( (x) * sizeof(HEntry) + HSHRDSIZE + (lenstr) )
+#define ARRPTR(x)       ( (HEntry*) ( (char*)(x) + HSHRDSIZE ) )
+#define STRPTR(x)       ( (char*)(x) + HSHRDSIZE + ( sizeof(HEntry) * ((HStore*)x)->size ) )
+
+
+#define PG_GETARG_HS(x)	((HStore*)PG_DETOAST_DATUM(PG_GETARG_DATUM(x)))
+
+typedef struct {
+        char    *key;
+        char    *val;
+        uint16  keylen;
+        uint16  vallen;
+        bool    isnull;  
+        bool    needfree;  
+} Pairs;
+
+int comparePairs(const void *a, const void *b);
+int uniquePairs(Pairs * a, int4 l, int4 *buflen);
+
+#endif