Improve equality concept & exercise

[verdammelt]

If possible work the concept & exercise such that this concept can be placed earlier in the track. Right now it depends upon strings and characters and arrays etc.

Perhaps with forward references to other concepts these pre-requisites can be removed.

[verdammelt]

The concept exericse mentions structure, but they are not actually used in the exercise so that reference could be removed.

The concept does mention pathnames etc - but they are in the about.md (except a single reference again to structures in the introduction (which is copied into the exercise introduction)) so this may be easy to clean up.