introduce internal Int128 type, use it to fix case stmt checks (#11652)

* initial version of int128
* use int128 in case stmt
* fixes #11552

tests/casestmt/tcasestmt.nim

@@ -226,3 +226,14 @@ block tcasestm:
         true
       else: raise newException(ValueError, "Invalid")
   ))
+
+#issue #11552
+
+proc positiveOrNegative(num: int): string =
+  result = case num
+  of (low(int)+2) .. -1:
+    "negative"
+  of 0:
+    "zero"
+  else:
+    "impossible"