It is unclear how to make data types with a type variable

Examples in plain Elixir:
 
 - [Maybe](https://github.com/serokellcao/etypes/blob/master/lib/etypes/types.ex#L6)
 - [Phantom](https://github.com/serokellcao/etypes/blob/master/lib/phantom.ex#L10)

Would be cool to do the same with `Either`/[`Result`](https://github.com/doma-engineering/cluster/blob/main/lib/uptight/result.ex#L14).