WIP draft of chars concept

[colinleach]

I decided there is enough in this to justify separating it from the Strings concept.

No decision yet on the corresponding exercise. I'll draft Strings first, then consider them both together.

[colinleach]

There are a couple of exercises in other tracks that might work;

• squeaky-clean (C#, F#, Clojure, Elm, Java)
• library-of-luton (Crystal)

I had wondered about putting Chars above Strings in the syllabus, to set the scene for discussing indexing issues in Strings. However, both these exercises include an element of string handling, and tracks with a separate Chars concept seem to put it low down.