Added int64 support to the script engine (#4522)

Added support for signed 64 bit integer value computation into the script engine.
This is required because newer official features require support for bigger numbers inside the scripts.

This also cleans up a lot of messy code and fixes a few issues like script stack corruptions.

Thanks to @aleos89 and everyone who supported me with this.

src/map/duel.cpp

@@ -63,7 +63,7 @@ void duel_savetime(struct map_session_data* sd)
  */
 bool duel_checktime(struct map_session_data* sd)
 {
-	int diff;
+	int64 diff;
 	time_t timer;
 	struct tm *t;