std.uni CodepointSet toString

[Carl Sturtivant]

Need help working around a linkage problem.
```d
import std.uni, std.conv, std.stdio, std.format;

void main() {
	//auto c1 = unicode.InBasic_latin;
	auto c1 = CodepointSet('a','z'+1);
	writeln(c1.to!string);
	writeln(format("%d", c1));
	writeln(format("%#x", c1));
	writeln(format("%#X", c1));
	writefln("%s", c1);
}
```
doesn't link, but does link with the commented out CodepointSet 
instead. Combines code from these examples at the following URLs.
https://dlang.org/phobos/std_uni.html#InversionList
https://dlang.org/phobos/std_uni.html#.InversionList.toString
```
$ dmd --version
DMD64 D Compiler v2.107.0
Copyright (C) 1999-2024 by The D Language Foundation, All Rights 
Reserved written by Walter Bright
$ dmd cset2.d
/usr/bin/ld: cset2.o: in function 
`_D4core8internal7switch___T14__switch_errorZQrFNaNbNiNfAyamZv':
cset2.d:(.text._D4core8internal7switch___T14__switch_errorZQrFNaNbNiNfAyamZv[_D4core8internal7switch___T14__switch_errorZQrFNaNbNiNfAyamZv]+0x19): undefined reference to `_D4core9exception__T15__switch_errorTZQsFNaNbNiNeAyamZv'
collect2: error: ld returned 1 exit status
Error: linker exited with status 1
```