This makes it possible to call public functions that take a string class
as a template parameter and return an object of that class (implemented
through the StringByteSink helper class) also with the CharString class,
even though this class doesn't actually provide the public API required
by StringByteSink.
This makes it possible to use such more modern APIs also internally.
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.
The repeated sequence of allocating a CheckedArrayByteSink, calling some
function that writes into this, then checking for overflow and returning
through u_terminateChars() can all be moved into a single shared helper
function.
