Improve Concept Exercise: strings

[ErikSchierboom]

Taking a look at the strings exercise, I've found some things that can be improved:

• The stub file does not contain stubs for the methods the student needs to implement. These should be added (see the docs).
• The example implementation file uses the const modifier. As per the docs, the example implementation should only use language features introduced by the exercise or its prerequisites (and their prerequisites, and so on). Is const introduced in one of the prerequisites?