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`