ObjFW  Check-in [7a488c3062]

This is the first step to have different versions of those for different
operating systems, rather than #ifdefs everywhere.

This also has the nice side-effect of not having one implementation in
OFString and another one in OFString_UTF8 anymore. The one in OFString
was a generic version, while the one in OFString_UTF8 was one optimized
for UTF-8 strings. Now only the version optimized for UTF-8 strings
exists, as this is by far the most common string implementation used,
and the overhead of converting from something else to UTF-8 is not more
than converting to UTF-32.