section 7.14 uses references

[wrigjl]

The problems in section 7.14 use string references which are not covered until 8.7. Solution: rewrite with pass-by-value + return.

• character_classification_AC_1
• character_classification_1
• character_classification_2

[wrigjl]

This issue comes from the original book. I've got a pull request filed with the author

[wrigjl]

fixed with 8abe5a4, 86a3397, and 51c545f