Split tuptoaster.c into three separate files.

detoast.c/h contain functions required to detoast a datum, partially
or completely, plus a few other utility functions for examining the
size of toasted datums.

toast_internals.c/h contain functions that are used internally to the
TOAST subsystem but which (mostly) do not need to be accessed from
outside.

heaptoast.c/h contains code that is intrinsically specific to the
heap AM, either because it operates on HeapTuples or is based on the
layout of a heap page.

detoast.c and toast_internals.c are placed in
src/backend/access/common rather than src/backend/access/heap.  At
present, both files still have dependencies on the heap, but that will
be improved in a future commit.

Patch by me, reviewed and tested by Prabhat Sabu, Thomas Munro,
Andres Freund, and Álvaro Herrera.

Discussion: http://postgr.es/m/CA+TgmoZv-=2iWM4jcw5ZhJeL18HF96+W1yJeYrnGMYdkFFnEpQ@mail.gmail.com

src/backend/executor/execExprInterp.c

@@ -56,7 +56,7 @@
  */
 #include "postgres.h"
 
-#include "access/tuptoaster.h"
+#include "access/heaptoast.h"
 #include "catalog/pg_type.h"
 #include "commands/sequence.h"
 #include "executor/execExpr.h"

src/backend/executor/execTuples.c

@@ -57,9 +57,9 @@
  */
 #include "postgres.h"
 
+#include "access/heaptoast.h"
 #include "access/htup_details.h"
 #include "access/tupdesc_details.h"
-#include "access/tuptoaster.h"
 #include "funcapi.h"
 #include "catalog/pg_type.h"
 #include "nodes/nodeFuncs.h"

src/backend/executor/tstoreReceiver.c

@@ -20,7 +20,7 @@
 
 #include "postgres.h"
 
-#include "access/tuptoaster.h"
+#include "access/detoast.h"
 #include "executor/tstoreReceiver.h"