The repeated sequence of allocating a CharString and CharStringByteSink,
before calling some function that writes into this, can be moved into a
single shared helper function which then is used to give all ulocimp.h
functions that write to ByteSink an overload that instead returns a
CharString, to make call sites look like perfectly normal C++ code.
· No function should do anything if an error has already occurred.
· On error, a value of 0, nullptr, {}, etc., should be returned.
· Values shouldn't have overloaded meanings (eg. index or found).
· Values that are never used should not be returned at all.
Some of this code was originally written as C code and some of this code
was originally written as C++ code but made to resemble the then already
existing code that had once been C code. Changing it all to normal C++
now will make it easier and safer to work with going forward.
· Use unnamed namespace instead of static.
· Use reference instead of non-nullable pointer.
· Use bool instead of UBool.
· Use constexpr for static data.
· Use U_EXPORT instead of U_CAPI or U_CFUNC.
· Use the default calling convention instead of U_EXPORT2.
