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)
You would specify the type with a suffix:

```rust
match x {
   1u => { ... },
   2i => { ... },
   _ => { ... }
}
```