Phobos' std.conv.to-conversion from enum to string doesn't scale beyond hundreds of enumerators

[Per Nordlöw]

I've discovered the annoying fact that std.conv.to doesn't scale 
for enum to string conversion when the enum has hundreds of 
members. This because of a call to `NoDuplicates` which has (at 
least) O(n*log(n) time and space complexity.

So I've come up with


/** Faster implementation of `std.conv.to`.
  */
string toString(T)(T value) @safe pure nothrow @nogc
if (is(T == enum))
{
     final switch (value)
     {
         static foreach (member; __traits(allMembers, T))
         {
         case __traits(getMember, T, member):
             return member;
         }
     }
}

///
@safe pure nothrow @nogc unittest
{
     enum E { unknown, x, y, z, }
     assert(E.x.toString == "x");
     assert(E.y.toString == "y");
     assert(E.z.toString == "z");
}


The question know is: How do I make this support enums with 
enumerator aliases without needing to call `NoDuplicates`?

For instance, this should work

///
@safe pure nothrow @nogc unittest
{
     enum E { unknown, x, y, z, z_ = z, }
     assert(E.x.toString == "x");
     assert(E.y.toString == "y");
     assert(E.z.toString == "z");
     assert(E.z_.toString == "z");
}