Support gen syntax (feature gen_blocks)

[mohe2015]

Hi, it would be nice if the following would parse and show types correctly.

#![feature(gen_blocks)]

pub async gen fn html() {
    let _ = async gen { yield (); };
    yield ();
}

It looks like there already is support for coroutines

#![feature(coroutines)]

pub fn html() {
    let test = || { yield (); };
}

so maybe this is not too hard.

[mohe2015]

I'm currently experimenting a bit on this and using this as an opportunity to get to know the code of rust-analyzer