Voldemort toHexString Compile Error

[Salih Dincer]

```d
void main()
{
   import std.stdio;

   "Hello D".toHexString.writeln;
}

struct HexString
{
   int i;

   auto nibbleSplit()
   {
     auto MSB = i >> 4;
     auto LSB = i & 15;
     return [MSB, LSB];
   }
}

auto toHexString(string str)
{
   struct Hex { int i;
     auto nibbleSplit() {
       auto MSB = i >> 4;
       auto LSB = i & 15;
       return [MSB, LSB];
     }
   }

   import std.algorithm : map;
   import std.conv, std.range : join;

   return str.map!Hex//String
             .map!(e => e.nibbleSplit).join
             .map!(n => n += n > 9 ? '7' : '0')
             .map!(c => c.to!char);
}
```

Why doesn't the function compile?  But the code is compiled with 
the HexString structure.  The error given by the compiler is as 
follows:

> /dlang/dmd/linux/bin64/../../src/phobos/std/algorithm/iteration.d(582): Error: cannot access frame pointer of `onlineapp.toHexString.Hex`
/dlang/dmd/linux/bin64/../../src/phobos/std/algorithm/iteration.d(479): Error: template instance `std.algorithm.iteration.MapResult!(Hex, string)` error instantiating
onlineapp.d(33):        instantiated from here: `map!string`
onlineapp.d(34): Error: template `onlineapp.toHexString.map!((e) 
=> e.nibbleSplit).map` is not callable using argument types 
`!()(MapResult!(Hex, string))`
/dlang/dmd/linux/bin64/../../src/phobos/std/algorithm/iteration.d(446):        Candidate is: `map(Range)(Range r)`
   with `Range = MapResult!(Hex, string)`
   must satisfy the following constraint:
`       isInputRange!(Unqual!Range)