Self-documenting `offer!()` and `choose!()`

Could we do something like:

```rust
offer!(in chan {
    End => break chan.close(),
    Push => {
        // push a value
    }
})?;
```

`End` and `Push` would reduce to enums:

```rust
enum Choice {
    End,
    Push
}

offer!(in chan {
    Choice::End => ...,
    Choice::Push => ...,
})?;
```