4x faster strlen with 4 char sentinel

[Jay Norwood via Digitalmars-d-announce]

After watching Andre's sentinel thing, I'm playing with strlen on 
char strings with 4 terminating 0s instead of a single one.  
Seems to work and is 4x faster compared to the runtime version.

nothrow pure size_t strlen2(const(char)* c) {
  if (c is null)
    return 0;
  size_t l=0;
  while (*c){ c+=4; l+=4;}
  while (*c==0){ c--; l--;}
  return l+1;
}

This is the timing of my test case, which I can post if anyone is 
interested.
strlen\Release>strlen
2738
681