add ToPrimitive to int.Int driver prelude

drivers/rust.drv

@@ -13,7 +13,8 @@ theory Bool
 end
 
 module int.Int
-  prelude "use num::bigint::BigInt;"
+  prelude "use num::bigint::BigInt;
+use num::cast::ToPrimitive;"
   syntax constant zero "BigInt::zero()"
   syntax constant one "BigInt::one()"