Use #[non_exhaustive] where applicable

[LikeLakers2]

Hi there! This changes all of those odd __Nonexhaustive enum cases, so that they instead use the #[non_exhaustive] attribute.

However, I opted not to add the #[non_exhaustive] attribute to any structs where it could apply (that is, those who had a __nonexhaustive: () field), since I'm unsure if the effects would be acceptable:

Non-exhaustive structs could have additional fields added in future. Therefore, non-exhaustive structs cannot be constructed in external crates using the traditional Struct { .. } syntax; cannot be matched against without a wildcard ..; and struct update syntax will not work.

That said, if it's desired that I add it to any applicable structs, I can do so.

Side note: I did notice #752 while searching to see if there was an existing PR that made this change -- but being that the PR is from December 2019, I imagine there would be lots of merge conflicts and other misc issues that would make it hard to merge that. So I decided to make my own PR.

[arqunis]

since I'm unsure if the effects would be acceptable

The private field enforces the effects already. The attribute just makes it more apparent to the user and the compiler that a struct/enum is non-exhaustive, thus tooling can support the fact (e.g., rustdoc showing it in documentation). You should switch the structs to use the attribute as well.

[LikeLakers2]

@acdenisSK I've added #[non_exhaustive] to any applicable structs, as you requested.