bigfile/py: Move PyVMA's support for cyclic GC close to pyvma_dealloc

The logic in pyvma_traverse and pyvma_clear needs to be synchronized
with PyVMA deallocation. In the next patche we'll be amending this
logic, and it will help a reader to keep all those functions together.

For the reference: PyVMA support for cyclic GC was introduced in
450ad804 (bigarray: ArrayRef support for BigArray). See also d97641d2
(bigfile/py: Properly untrack PyVMA from GC before dealloc).

/proposed-for-review-on !12

bigfile/_bigfile.c

 /* Wendelin.bigfile | Python interface to memory/files
- * Copyright (C) 2014-2019  Nexedi SA and Contributors.
+ * Copyright (C) 2014-2020  Nexedi SA and Contributors.
  *                          Kirill Smelkov <kirr@nexedi.com>
  *
  * This program is free software: you can Use, Study, Modify and Redistribute
@@ -165,26 +165,6 @@ pyvma_len(PyObject *pyvma0)
 }
 
 
-/* pyvma vs cyclic GC */
-static int
-pyvma_traverse(PyObject *pyvma0, visitproc visit, void *arg)
-{
-    PyVMA *pyvma = container_of(pyvma0, PyVMA, pyobj);
-
-    Py_VISIT(pyvma->pyuser);
-    return 0;
-}
-
-static int
-pyvma_clear(PyObject *pyvma0)
-{
-    PyVMA *pyvma = container_of(pyvma0, PyVMA, pyobj);
-
-    Py_CLEAR(pyvma->pyuser);
-    return 0;
-}
-
-
 PyFunc(pyvma_filerange, "filerange() -> (pgoffset, pglen) -- file range this vma covers")
     (PyObject *pyvma0, PyObject *args)
 {
@@ -211,6 +191,26 @@ PyFunc(pyvma_pagesize, "pagesize() -> pagesize -- page size of RAM underlying th
 }
 
 
+/* pyvma vs cyclic GC */
+static int
+pyvma_traverse(PyObject *pyvma0, visitproc visit, void *arg)
+{
+    PyVMA *pyvma = container_of(pyvma0, PyVMA, pyobj);
+
+    Py_VISIT(pyvma->pyuser);
+    return 0;
+}
+
+static int
+pyvma_clear(PyObject *pyvma0)
+{
+    PyVMA *pyvma = container_of(pyvma0, PyVMA, pyobj);
+
+    Py_CLEAR(pyvma->pyuser);
+    return 0;
+}
+
+
 static void
 pyvma_dealloc(PyObject *pyvma0)
 {