Add missing Elixir structs

[colinsmetz]

Many of the modules defined in datatypes.rs do not have an equivalent struct in the Elixir code, so we receive structs that Elixir does not recognize and have to match explicitly on %{__struct__: module, ...} which is a bit annoying.

Is it possible to add them?

Also, would you prefer to add tests for each of them before adding them?

[maartenvanvliet]

Thanks! Test cases for each of them are not necessary imo. I'd rather see some kitchen sink integration tests eventually. And some property tests to see if the sql -> rust -> elixir -> rust -> sql conversion works out.