[concept] Cleanup isCallable()
Now isCallable(f,args...)
checks if f(args...)
is valid. Before,
the arguments had to be given as a Dune::TypeList
which is misleading
and can even be considered a bug. Since this was not used anywhere,
this commit changes the behaviour to the expected one.
This also fixes the return value type of isCallable()
by
properly forwarding bool_constant
values.