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/Misc/NEWS b/Misc/NEWS
index c4aba0d..67227b1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@
 Core and Builtins
 -----------------
 
+- Issue #12911: Fix memory consumption when calculating the repr() of huge
+  tuples or lists.
+
 - Issue #7732: Don't open a directory as a file anymore while importing a
   module. Ignore the direcotry if its name matchs the module name (e.g.
   "__init__.py") and raise a ImportError instead.