Forward declaration of extension class in another module

Forward declaration of extension class in another module

+
+    - There is now a way of forward-declaring an extension type into
+      another module. This allows two .pxd files to define extension types
+      that refer to each other without running into circular import problems.
+      For example:
+
+        cimport blarg
+        cdef class blarg.Blarg # Forward declaration
+
+        cdef class Foo:
+          cdef blarg.Blarg blg