SWI-cpp2.h should be in a namespace

[kamahen]

The reason is given here: https://google.github.io/styleguide/cppguide.html#Namespaces

This would require users of SWI-cpp2.h to add a "using namespace swi_cpp2;" or add "swi_cpp2::" to all references to items in SWI-cpp2.h.

[JanWielemaker]

Just swi?

[kamahen]

Namespace swi seems good to me. I'll prepare some PRs but I'm not in a hurry.