toggleCase() for Narrow String

[Salih Dincer]

Hi,

Please watch: https://youtu.be/siw602gzPaU

If he had learned this method like me, he would not have needed 
if's and long explanations. :)

It's a very redundant thing (ASCII magic) that also weeds out the 
incompatible character set. This approach adopts the toggle/flip 
method through XOR rather than toLower/toUpper:

```d
import std.traits : isNarrowString;
bool isAsciiString(T)(T str)
if (isNarrowString!T)
{
   import std.uni : byGrapheme;
   auto range = str.byGrapheme;

   import std.range : walkLength;
   return range.walkLength == str.length;
}

auto toggleCase(T)(T str)
in (str.isAsciiString, "Incompatible character set!")
{
   auto ret = str.dup;
   foreach (ref chr; ret) {
     chr ^= ' '; // toggle character
   }
   return ret;
}

import std.stdio;
void main()
{
   string test = "dlANG"; // "ḍlANG";

   test.toggleCase().writeln(); // "DLang"
}
```

SDB at 79