4x faster strlen with 4 char sentinel

[Jay Norwood via Digitalmars-d-announce]

On Tuesday, 28 June 2016 at 01:53:22 UTC, deadalnix wrote:
> If we were in interview, I'd ask you "what does this returns if 
> you pass it an empty string ?"

I'd say use this one instead, to avoid negative size_t. It is 
also a little faster for the same measurement.

nothrow pure size_t strlen2(const(char)* c) {
   if (c is null)
      return 0;
   const(char)* c_save = c;
   while (*c){ c+=4; }
   while (*c==0){ c--; }
   c++;
   return c - c_save;
}

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