- Reference is not using __call__ now, so (future) member function reference via proxy will be possible
- Reference now holds a 'numeric' property, removing the NumericReferenceMixin and the double class per reference type
- fixed tiny safe_repr bugs and a FieldOrAttr call bug
- added support for function calls from self_ref, you can now do:

        class Foo(Buffer):
            a = be_int_field(where=bytes_ref[0:1])
            s = str_field(where=bytes_ref[1:1 + num_ref(self_ref.calc_s_size(a))])

            def calc_s_size(self, a):
                return a * 2