Hasten is_integer

the commented further improvement should be done too imo, but up to you

game/scripts/utility/common_functions.rpy

@@ -127,14 +127,15 @@ init python early:
             return obj
 
     def is_integer(s):
-        def zero(s):
-            return (len(s) > 1 and s.startswith("0"))
-
         s = str(s)
-
-        if s and s[0] in ("-", "+"):
-            return (not zero(s[1:]) and s[1:].isdigit())
-        return (not zero(s) and s.isdigit())
+        if not s:
+            return False
+        if s[0] in ("-", "+"):
+            # calling lstrip("0+-") would be faster but not exactly identical
+            s = s[1:]
+        if s.lstrip("0").isdigit():
+            return True
+        return False
 
     def timeit(func, loops=10000, args=(), kwargs={}):
         start = time.time()