ObjFW  Check-in [b7b33247d6]

This is only useful for size-constrained systems and changes the
behavior as follows:

* Case-insensitive comparisons only work as expected for ASCII
characters. This means that while a and A compare equal, ä and Ä do
not.

* -[OFMutableString {upper,lower}] and
-[OFString {upper,lower}caseString] only work on ASCII characters;
all other characters are left as they are. This means that the upper
version of a is A, but the upper version of ä is still ä (and vice
versa for lower).