Arabic Normalizer (c++)  + Buckwalter Romanization Converter

(This pull request is for reviewing)


Rather than dummy initialise `subst_ch` to 0 and then assign to it on the next line, it would be more usual to initialise directly with the wanted value, like so:

```
    unsigned ch = *i;
    unsigned subst_ch = get_value(ch, mapping, mapping_length);
```