The way they were being defined inside the `included` hook meant
they ended up being defined directly on TableDefinition, contrary
to the documentation:
https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/PostgreSQL/ColumnMethods.html

I left `define_column_methods` as a class method that gets extended
onto TableDefinition, since there's at least one gem that relies
on being able to call it there to define their own column types