class JoinCallbacks

Supports joining a sequence of callbacks (with one parameter only) into a single callback that

takes a vector of their parameters. The resulting vector will be in order that the callbacks

were CREATED (not issued).

Supporting multiple parameters for each callback adds significant template complexity and

requires us either to store a std::tuple (which can be difficult to use) or have the caller

provide some container type (difficult to use in a different way). It also makes the common case

of one parameter more difficult (or we need even more template specializations).

If you need multiple parameters, it's recommended you wrap the callback to pack the parameters

into a struct and then pass that struct to the callback provided by this class.