Fails due to Fixnum removal after upgrading to Ruby 3.2

[hwo411]

Ruby 3.2 removed Fixnum and Sequel bit fields now fails with uninitialized constant Sequel::Plugins::BitFields::Fixnum.

https://github.com/sdepold/sequel-bit_fields/blob/master/lib/sequel/plugins/bit_fields.rb#L82

We need to change it to Integer, wonder if we should release next major version and require Ruby > 2.4 or just conditionally wrap this part to use Integer after Ruby 2.4 and Fixnum before.