Support `gen` syntax (feature gen_blocks)

Hi, it would be nice if the following would parse and show types correctly.
```rust
#![feature(gen_blocks)]

pub async gen fn html() {
    let _ = async gen { yield (); };
    yield ();
}
```
It looks like there already is support for coroutines
```rust
#![feature(coroutines)]

pub fn html() {
    let test = || { yield (); };
}
```
so maybe this is not too hard.