use sealed traits for State traits

[DanGould]

@0xbeefcaf3

Originally posted by @DanGould in #728 (comment)

[arminsabouri]

Original Comment for more context

[mehmetefeumit]

@DanGould I can take a look at this. It's nicely related to the section I updated the documentation for.

[DanGould]

yes please do