Consider add support of recently merged immovable generators

[andreytkachenko]

Recently immovable generator PR #45337 has been finally merged into the rust's master branch and it is now possible use it in the latest nightly builds.

[alexcrichton]

I'd be totally down for a PR along these lines!

[andreytkachenko]

I am also don't like unsafe approach, but it makes possible share generators state.
here is an example:

#[async]
fn task(pin: &gpio::Pin) -> Result<(), uart::UartError> {
    let mut buff = [0u8; 16];
    loop {
        await!(uart::read_until(&mut buff, '\n'))?; // without static generators 
                                                    // this will cause a compilation error
        match buff {
            b"on" => pin.set_high(),
            b"off" => pin.set_low(),
            _ => {}
        }
    }
} 

[withoutboats]

These are now supported :)