Add a "Free pools" column to "heap cpython-allocators" output

Indicate proportion of memory within the allocator that's
currently unused.

Ideally we'd also give stats comparing:
  - never touched
  - in use
  - freed
but for now we just show:
  avail / total

heap/cpython.py

@@ -589,10 +589,13 @@ def __init__(self):
 
     @need_debuginfo
     def invoke(self, args, from_tty):
-        t = Table(columnheadings=('struct arena_object*', '256KB buffer location'))
+        t = Table(columnheadings=('struct arena_object*', '256KB buffer location', 'Free pools'))
         for arena in ArenaObject.iter_arenas():
             t.add_row([fmt_addr(arena.as_address()),
-                       fmt_addr(arena.address)])
+                       fmt_addr(arena.address),
+                       '%i / %i ' % (arena.field('nfreepools'),
+                                     arena.field('ntotalpools'))
+                       ])
         print 'Objects/obmalloc.c: %i arenas' % len(t.rows)
         t.write(sys.stdout)
         print

selftest.py

@@ -575,7 +575,9 @@ def _impl_test_python(self, pyruntime, py3k):
         # Verify that "cpython-allocators" works:
         allocators_out = tables[0]
         self.assertEquals(allocators_out.colnames,
-                          ('struct arena_object*', '256KB buffer location'))
+                          ('struct arena_object*',
+                           '256KB buffer location',
+                           'Free pools'))
 
         # print allocators_out
         # self.assertHasRow(allocators_out,