Rounding gives incorrect result for large numbers

As of version 1.5.8 it seems the largest result that the `round` function returns is 18446744073709551616 (aka. 2^64) and if the input is larger than that it just returns 18446744073709551616 instead. This applies to `floor` and `ceil` as well. Example: ``` > a=50!/e approx. 11188719610782480506113237301990711351094883421697948286614915791.4883462282 > round(a) approx. 18446744073709551616 # expected result is 11188719610782480506113237301990711351094883421697948286614915791 ```