These helper functions allow to create overload sets from lambdas.
E.g. one can merge to lambdas for different argument types into
one overload set in order to easily implement type-specific behaviour
when looping over a tuple.

While the result of overload(f1, f2,...) will pick the best match
for a given set of arguments, orderedOverload(f1,f2,...) will pick
the first match which allows to prevent ambiguity.

This functionality is proposed for the standard in P0051R2
(open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0051r2.pdf).
There overload(...) is proposed and an upcomming proposal
with the functionality of orderedOverload(...) is announced.
Since the former is not accepted and the latter is not even written
I did explicitly implement this in Dune:: instead of Dune::Std::.