new string mixins -- debug printfs

[Walter Bright]

BCS wrote:
> Bill Baxter wrote:
>> Here's something fun that's now possible:
>>
>> version(trace) {
>>     template Trace(char[] msg="") {
>>         const char[] Trace =
>>            `writefln("%s(%s): `~msg~`", __FILE__, __LINE__);`;
> 
> i'd use
> 
> `writefln(__FILE__"("~itoa!(__LINE__)~"): %s), "`~msg~`");`;
> 
> file and line get folded and % dosn't mess thing up
> 
> COOL

or:

  const char[] Trace =
   std.metastrings.Format!(
        `writefln("%%s", "%s(%s): %s")`,
        __FILE__, __LINE__, msg
      );