Allow different containers for gradeschool

[elyashiv]

In grade school the container is chosen to be map<int, vector<string>> a better choice would be std::set<string> or std::multiset<string> instead of vector, because the list of students needs to stay sorted.

I see two ways to fix this:

1. Change the tests to expect a std::set or std::mutliset
2. Change the tests accept any type of container, as long as it compiles. We can do this using the std::result_of trait.

The second approach gives a student more room to play with things, while it can make it harder to understand what fails to compile/what the tests actually check. It will also require a better testing framework, as we would want to test a few examples.

This might be true also for other exercises.