Refactor ss_toi() to not use stringstreams

Apart from the std::string, this function no longer uses the STL.

ss_toi() is a simple function - it converts the input into an int,
taking as many digits as possible until it reaches a non-digit
character, at which point it stops. It's trivial to implement this
without the STL.

I could've used Int() here, but that would've required copying the
string to a temporary buffer to insert a null-terminator (we can't just
use a pointer-and-length data type either, the string functions don't
operate like that - one disadvantage of C strings!). Instead, I decided
to implement my own conversion to int here, because I don't think the
way we humans write our Arabic numerals is going to change anytime soon.

Also, the std::string input is now passed by const reference, instead of
making a copy - cutting down on unnecessary memory operations.

desktop_version/src/UtilityClass.h

@@ -5,7 +5,7 @@
 #include <string>
 #include <vector>
 
-int ss_toi(std::string _s);
+int ss_toi(const std::string& str);
 
 std::vector<std::string> split(const std::string &s, char delim, std::vector<std::string> &elems);