CallableTraitsHelper identifies the return type and argument types of a
callable type and passes those to an implementation class template to
operate on.
The CallableArgInfo utility uses CallableTraitsHelper to provide
typedefs for the return type and argument types (as a tuple) of a
callable type.
In WrapperFunction.h, the detail::WFCallableTraits utility is rewritten
in terms of CallableTraitsHandler (and renamed to WFHandlerTraits).
