Add PYBIND11_EXPAND_SIDE_EFFECTS macro

This allows calling of functions (typically void) over a parameter
pack, replacing usage such as:

    bool unused[] = { (voidfunc(param_pack_arg), false)..., false };
    (void) unused;

with a much cleaner:

    PYBIND11_EXPAND_SIDE_EFFECTS(voidfunc(param_pack_arg));