Issue #12911: Fix memory consumption when calculating the repr() of huge tuples or lists.

This introduces a small private API for this common pattern.
The issue has been discovered thanks to Martin's huge-mem buildbot.
diff --git a/Makefile.pre.in b/Makefile.pre.in
index a2b14a9..cde3af0 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -342,6 +342,7 @@
 # Objects
 OBJECT_OBJS=	\
 		Objects/abstract.o \
+		Objects/accu.o \
 		Objects/boolobject.o \
 		Objects/bytes_methods.o \
 		Objects/bytearrayobject.o \
@@ -664,6 +665,7 @@
 		Include/Python-ast.h \
 		Include/Python.h \
 		Include/abstract.h \
+		Include/accu.h \
 		Include/asdl.h \
 		Include/ast.h \
                 Include/bltinmodule.h \