Arabic Normalizer (c++)  + Buckwalter Romanization Converter

(This pull request is for reviewing)


Inside the library, it's preferable to include just the sub-headers you actually need (e.g. `xapian/unicode,h`).

This greatly reduces the amount of recompiling needed when an API header is changed (if we just included `xapian.h` everywhere, then touching any API header would require almost everything to be recompiled).

OTOH, people using the library should just `#include <xapian.h>` (at least currently - perhaps if the library grows to the point where the header processing time becomes an issue we might split things up from a user point of view).