Factorial: Use `Integer` instead of `Int` for bigger numbers

The `Integer` type has arbitrary precision. This is why the function was
working fine in GCHi but overflowing when I was testing the bot. Use
`Integer` to get the real values we want.

https://stackoverflow.com/questions/36355757/large-numbers-become-negative-in-haskell

Also add a little extra error handling to deal with unexpected numbers.
Now that I think about it, the regex parser isn't going to handle
negative numbers, so we're good on that front. Set an upper bound so we
don't end up spending a lot of time trying to calculate numbers and
potentially segfault if huge numbers are given.

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