Move Trigger and TriggerDesc structs out of rel.h into a new reltrigger.h

This lets us stop including rel.h into execnodes.h, which is a widely
used header.

src/backend/access/gin/ginfast.c

@@ -24,6 +24,7 @@
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
 #include "utils/memutils.h"
+#include "utils/rel.h"
 
 
 #define GIN_PAGE_FREESIZE \

src/backend/access/gin/gininsert.c

@@ -21,6 +21,7 @@
 #include "storage/smgr.h"
 #include "storage/indexfsm.h"
 #include "utils/memutils.h"
+#include "utils/rel.h"
 
 
 typedef struct

src/backend/access/gist/gist.c

@@ -22,6 +22,7 @@
 #include "storage/bufmgr.h"
 #include "storage/indexfsm.h"
 #include "utils/memutils.h"
+#include "utils/rel.h"
 
 /* Working state for gistbuild and its callback */
 typedef struct

src/backend/access/gist/gistget.c

@@ -22,6 +22,7 @@
 #include "storage/bufmgr.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
+#include "utils/rel.h"
 
 
 /*

src/backend/access/hash/hash.c

@@ -25,6 +25,7 @@
 #include "optimizer/cost.h"
 #include "optimizer/plancat.h"
 #include "storage/bufmgr.h"
+#include "utils/rel.h"
 
 
 /* Working state for hashbuild and its callback */

src/backend/access/hash/hashutil.c

@@ -20,6 +20,7 @@
 #include "executor/execdebug.h"
 #include "storage/bufmgr.h"
 #include "utils/lsyscache.h"
+#include "utils/rel.h"
 
 
 /*

src/backend/access/nbtree/nbtutils.c

@@ -27,6 +27,7 @@
 #include "storage/lwlock.h"
 #include "storage/shmem.h"
 #include "utils/lsyscache.h"
+#include "utils/rel.h"
 
 
 static bool _bt_compare_scankey_args(IndexScanDesc scan, ScanKey op,