Put some extensions into their own namespaces

[jherico]

For instance, putting XR_EXT_debug_utils definitions into xr::ext::DebugUtils would mean that a C++ client would be able to write xr::ext::debugUtils::Messenger instead of xr::DebugUtilsMessengerEXT. Since setup and teardown will likely be using many related classes, this allows clients to declare a scoped using namespace statement and produce significantly less verbose code.