Merge the tim-obmalloc branch to the trunk.

This is a heavily altered derivative of SF patch 1123430, Evan
Jones's heroic effort to make obmalloc return unused arenas to
the system free(), with some heuristic strategies to make it
more likley that arenas eventually _can_ be freed.

Misc/NEWS

@@ -12,6 +12,18 @@ What's New in Python 2.5 alpha 1?
 Core and builtins
 -----------------
 
+- Patch #1123430: Python's small-object allocator now returns an arena to
+  the system ``free()`` when all memory within an arena becomes unused
+  again.  Prior to Python 2.5, arenas (256KB chunks of memory) were never
+  freed.  Some applications will see a drop in virtual memory size now,
+  especially long-running applications that, from time to time, temporarily
+  use a large number of small objects.  Note that when Python returns an
+  arena to the platform C's ``free()``, there's no guarantee that the
+  platform C will in turn return that memory to the operating system.  The
+  effect of the patch is to stop making that impossible, and in tests it
+  appears to be effective at least on Microsoft C and gcc-based systems.
+  Thanks to Evan Jones for hard work and patience.
+
 - Patch #1434038: property() now uses the getter's docstring if there is
   no "doc" argument given. This makes it possible to legitimately use
   property() as a decorator to produce a read-only property.