Auto merge of #118457 - eholk:genfn, r=compiler-errors

Add support for `gen fn`

This builds on #116447 to add support for `gen fn` functions. For the most part we follow the same approach as desugaring `async fn`, but replacing `Future` with `Iterator` and `async {}` with `gen {}` for the body.

The version implemented here uses the return type of a `gen fn` as the yield type. For example:

```rust
gen fn count_to_three() -> i32 {
    yield 1;
    yield 2;
    yield 3;
}
```

In the future, I think we should experiment with a syntax like `gen fn count_to_three() yield i32 { ... }`, but that can go in another PR.

cc `@oli-obk` `@compiler-errors`

Author: bors