Tagged unions with tag-only members

[emil14]

Mixed

This will allow to have maybe as a union

type maybe<T> union {
    Some T
    None
}

And maybe will simplify/improve design for #747 because it's no longer special case

Tag-only

Since it's possible to have tag-only members, it's possible to have tag-only unions

type Weekdays union {
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
}

Tags are (most likely) gonna be represented with 1-byte integers, same as enums. This is basically 2 ways of doing the same thing

This is kinds the same thing value unions in TypeScript (despite better memory consumption):

type Weekdays = 'Monday'
    | 'Tuesday'
    | 'Wednesday'
    | 'Thursday'
    | 'Friday'

Naming?

This is basically what's called "enums" in Rust. I don't like that name, honestly, since it's more related to C-like enums to me, but we'll see.

[emil14]

Should be part of #751