[LWG #280] std::reverse_iterator missing template assignment operator

LWG Issue 280 added a template assignment operator to reverse_iterator:
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#280

It doesn't look like we ever added the function.

I'm having trouble coming up with a test case that would reveal its absence so this is just a placeholder to remind us to look into this in more detail to make sure the function really is needed (if not, there's no need to complicate the interface of the template).