Use templetized operations for File and Rank

Doing the conversion the compiler is now able to
spot two possible ambiguity calls that now we can
easily fix.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/position.cpp

@@ -182,7 +182,7 @@ void Position::from_fen(const string& fen) {
   {
       if (isdigit(token))
       {
-          file += token - '0'; // Skip the given number of files
+          file += File(token - '0'); // Skip the given number of files
           continue;
       }
       else if (token == '/')