Auto merge of #116447 - oli-obk:gen_fn, r=compiler-errors

Implement `gen` blocks in the 2024 edition

Coroutines tracking issue https://github.com/rust-lang/rust/issues/43122
`gen` block tracking issue https://github.com/rust-lang/rust/issues/117078

This PR implements `gen` blocks that implement `Iterator`. Most of the logic with `async` blocks is shared, and thus I renamed various types that were referring to `async` specifically.

An example usage of `gen` blocks is

```rust
fn foo() -> impl Iterator<Item = i32> {
    gen {
        yield 42;
        for i in 5..18 {
            if i.is_even() { continue }
            yield i * 2;
        }
    }
}
```

The limitations (to be resolved) of the implementation are listed in the tracking issue

compiler/stable_mir/src/mir/body.rs

@@ -187,6 +187,7 @@ pub enum UnOp {
 pub enum CoroutineKind {
     Async(CoroutineSource),
     Coroutine,
+    Gen(CoroutineSource),
 }
 
 #[derive(Clone, Debug)]