Allow runtime collections of setups to be passed to runBenchmarks

[chillenzer]

The current interface is a bit annoying to use. It is targeted at very different kinds of recipes that are best expressed in separate types. But a standard usecase will likely be to have a (runtime-dependent) number of each of those recipes that is parametrised (at runtime!). An interface like this would probably be convenient:

std::vector<int> numbers{1,2,3,4};
std::vector<Setup> setups1 = composeSetups(numbers);
auto setup2 = composeOtherSetup();
auto report = runBenchmarks(setups1, setup2);