Instead of only printing "BUG" let's print information about objects
which still refer to pybuf - to help debugging.

For example with the following artificial pybuf leak

```
diff --git a/bigfile/tests/test_basic.py b/bigfile/tests/test_basic.py
index c737621..f5e057a 100644
--- a/bigfile/tests/test_basic.py
+++ b/bigfile/tests/test_basic.py
@@ -126,6 +126,7 @@ def test_basic():

 # test that python exception state is preserved across pagefaulting
 def test_pagefault_savestate():
+    zzz = []
     class BadFile(BigFile):
         def loadblk(self, blk, buf):
             # simulate some errors in-between to overwrite thread exception
@@ -154,6 +155,7 @@ def loadblk(self, blk, buf):
             # which result in holding additional ref to buf, but loadblk caller
             # will detect and handle this situation via garbage-collecting
             # above cycle.
+            zzz.append(buf)

             self.loadblk_run = 1
```

it dies this way:

    bigfile/_bigfile.c:567 pybigfile_loadblk WARN: pybuf->ob_refcnt != 1 even after GC:
    pybuf (ob_refcnt=2):    <read-write buffer ptr 0x7f08d3e88000, size 2097152 at 0x7f08d48b7070>
    pybuf referrers:        [[<read-write buffer ptr 0x7f08d3e88000, size 2097152 at 0x7f08d48b7070>]]
    bigfile/_bigfile.c:573 pybigfile_loadblk        BUG!