Default TOAST re-implemented using TOAST API.

Default TOAST mechanics extracted from Heap AM Core and
re-implemented using Pluggable TOAST API as 'Default Toaster'.
Default toaster is added as initial entry in PG_TOASTER table,
with Toaster name 'deftoaster' and still is part of PostgreSQL
core.

Overall TOAST mechanics is kept the same, the difference is
that from now on TOAST table initialization is done within
Generic Toaster "init" functions, toast and detoast functions
are called as Toaster with DEFAULT_TOASTER_OID via TOAST API.
Default Toaster is attached to TOASTable column by default if
not stated otherwise (by CREATE or ALTERD TABLE command option
SET TOASTER). Default Toaster could be replaced with custom
toaster by SET TOASTER clause in CREATE TABLE or ALTER TABLE
command.

Author: Teodor Sigaev <teodor@sigaev.ru>
Author: Oleg Bartunov <obartunov@postgrespro.ru>
Author: Nikita Glukhov <n.gluhov@postgrespro.ru>
Author: Nikita Malakhov <n.malakhov@postgrespro.ru>

Discussion: https://www.postgresql.org/message-id/flat/224711f9-83b7-a307-b17f-4457ab73aa0a@sigaev.ru

Author: Nikita Malakhov

contrib/amcheck/verify_heapam.c

@@ -10,7 +10,7 @@
  */
 #include "postgres.h"
 
-#include "access/detoast.h"
+#include "access/toasterapi.h"
 #include "access/genam.h"
 #include "access/heapam.h"
 #include "access/heaptoast.h"
@@ -24,6 +24,8 @@
 #include "storage/procarray.h"
 #include "utils/builtins.h"
 #include "utils/fmgroids.h"
+#include "access/toast_helper.h"
+#include "catalog/toasting.h"
 
 PG_FUNCTION_INFO_V1(verify_heapam);
 

src/backend/access/Makefile

@@ -9,6 +9,6 @@ top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
 SUBDIRS	    = brin common gin gist hash heap index nbtree rmgrdesc spgist \
-			  table tablesample transam
+			  table tablesample toast transam
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/access/brin/brin_tuple.c

@@ -32,15 +32,15 @@
 #include "postgres.h"
 
 #include "access/brin_tuple.h"
-#include "access/detoast.h"
+#include "access/toasterapi.h"
 #include "access/heaptoast.h"
 #include "access/htup_details.h"
 #include "access/toast_internals.h"
 #include "access/tupdesc.h"
 #include "access/tupmacs.h"
 #include "utils/datum.h"
 #include "utils/memutils.h"
-
+#include "access/toast_helper.h"
 
 /*
  * This enables de-toasting of index entries.  Needed until VACUUM is

src/backend/access/common/Makefile

@@ -15,7 +15,6 @@ include $(top_builddir)/src/Makefile.global
 OBJS = \
 	attmap.o \
 	bufmask.o \
-	detoast.o \
 	heaptuple.o \
 	indextuple.o \
 	printsimple.o \