Add sequence() initializer to pass a runtime container of initializers.

[nat-goodspeed]

sequence() returns an initializer adapter that forwards initializer method
calls to each contained initializer in turn.

Also defines any_initializer as the element type for the container of
interest.

std::vector<any_initializer> initializers;
initializers.push_back(run_exe("myprog"));
...
execute(sequence(initializers));