SimpleString::StrCmp (2)

1. This implementation works
2. StrCmp and StrNCmp should be similarly implemented (whichever way)
3. More complete tests for both
4. @basvodde Not quite what you had in mind?
Can't this be:
```
for (; *s1 != 0; s1++, s2++)
```
Then you can delete like 71-73

Or am I seeing this wrong?