RFC: Anonymous enum types called joins, as `A | B`

Add support for defining anonymous, enum-like types using `A | B`.

[Rendered](https://github.com/reem/rfcs/blob/anonymous-enums/text/0000-anonymous-enums.md)
How is this solved for things like `x as A + B` being parseable as `(x as A) + B` or `x as (A + B)`.