Implement iterator api

A Maybe is a container like vector but has either 0 or 1 element.
This would be convenient:

```c++
for(auto x : maybe(1) ) {
  std::cout << x << std::endl; // prints 1
} 

for(auto x : maybe<int>() ) {
  //loop not entered
} 

```

Functional maps should never have sideeffects. For sideeffects this way should be preferred.