Non-static generators should always be unpin

[compiler-errors]

#![feature(generators, generator_trait)]

use std::marker::{PhantomPinned, Unpin};

fn assert_unpin<G: Unpin>(_: G) {
}

fn main() {
    // Even though this generator holds a `PhantomPinned` in its environment, it
    // remains `Unpin`.
    assert_unpin(|| {
        let pinned = PhantomPinned;
        yield;
        drop(pinned);
    });
}

Just needs special logic added to builtin impls.

[compiler-errors]

Conversely:

use std::marker::Unpin;

fn assert_unpin<T: Unpin>(_: T) {
}

fn main() {
    let mut generator = static || {
        yield;
    };
    assert_unpin(generator); //~ ERROR E0277
}

passes but it should not.