Added helper methods to LinksByKey. To be used with care

These methods were requested by @sstahl to ease usage of the class.

However I want to warn people that they are neither entirely safe (rely on internal sorting that is not checked) nor optimized (more allocations than needed). So people should prefer applyToLinks and applyToAllLinks over them.

I've actually revisited the places where I used applyToLinks so far and in not a single one (out of 12) I could use directly the new methods.