Support Rust's Generator feature

[avanhatt]

Rust has a newish Generator type.

We currently fail with unimplemented for this type:

            ty::Generator(_, _, _) => unimplemented!(),

This is used by the popular tokio crate.

[zhassan-aws]

Kani crashes on the following program:

#![feature(generators, generator_trait)]

use std::ops::{Generator, GeneratorState};
use std::pin::Pin;

fn main() {
    let mut generator = || {
        yield 1;
        return "foo"
    };

    match Pin::new(&mut generator).resume(()) {
        GeneratorState::Yielded(1) => {}
        _ => panic!("unexpected return from resume"),
    }
    match Pin::new(&mut generator).resume(()) {
        GeneratorState::Complete("foo") => {}
        _ => panic!("unexpected return from resume"),
    }
}

We should emit an error and exit gracefully until we support this feature.

[zhassan-aws]

Emit an error instead of crash, then remove the "must have" label, and switch "bug" to "feature".