Calling `sum_` on an `Int` column and trying to use the result as an `Int` leads to a marshal error

It appears that (at least in Postgresql), summing a list of `int`s gives back a `numeric`. So when you call `sum_` from esqueleto on an `Int` column and use the result as an `Int` you get a persist marshal error. IMO this should probably be solved on esqueleto's end, by either inserting an explicit cast or by restricting the return type of `sum_` to ones which will not raise a marshal error.