Simplestring::AtoI()

Move PlatformSpecificAtoI() to SimpleString.
Yes, I mean declare the bool on line 72 and the int result on line 74 (before the for loop).

In C++, it is a common practice to declare the variables close to use. This is useful as I can immediately see the scope of the variable. E.g. you declared result at like 67, so when reading the code I carefully paid attention to result from line 67-76, only to discover it isn't used.

These are minor things but will make your code a little bit better and more readable.