Debugging a recent cargo-outdated bug, it would have been nice not to
wonder whether a rustc version change in GitHub's runner image was a
contributing factor.

The generated code for function bodies produces an `unreachable_code`
warning if the function has a `!` return type.

The offending bit is:
```rust
if let Some(__ret) == None::<!> {
  // unreachable
}
```

This PR fixes the warning by simply adding `#[allow(unreachable_code)]`
to the return statement. It also adds a test for it.

This problem can only be triggered on nightly with the `never_type`
feature enabled, but it is easy to fix here.

Fix unreachable code warning for functions that return `!`

Since #254.