ns-3: Enhance WifiInformationElement and use in Wi-Fi module

This patch builds on that captured in Issue 898045 (which is a proposed change for Bug 881 in the ns-3 Bugzilla module) to make use of the WifiInformationElement object in the wifi module.

It includes some changes (improvements, I believe) to the way Serialize and Deserialize methods of a number of Wi-Fi and mesh objects work (specifically, changing to pass the iterator by reference) in order to simplify things for the caller.

The Ssid and SupportedRates classes are then converted to derive from WifiInformationElement.

The patch is based on ns-3-dev changeset 6194:5f8e99c4c456 as modified by the changes in Issue 898045.

I've deliberately excluded the necessary updates to the python bindings from the uploaded files in order to keep the view as simple as possible.