core, wifi: Add wifi namespace (Part 1)
Add a wifi namespace including all classes and enums in src/wifi/model.
Helpers are not included in the wifi namespace, similarly to the aodv module. This choice can be discussed.
Examples and tests import all wifi symbols (by having using namespace ns3::wifi
), unless they use a few wifi symbols.
Part 2 will consist of removing the wifi prefix from names of classes, structs and enums.