Free function `yield_` is unsound

[jonas-schievink]

It can be used to transmute types. Example causing a segmentation fault (at least on my machine):

use generator::*;

fn main() {
    let g = Gn::new_scoped(|_| {
        yield_::<String, _>(());
    });

    for () in g {}
}

[Xudong-Huang]

yied_ should be deprecated. we should always use scoped version's yield method.

[Xudong-Huang]

now if call deprecated yield_ in a scoped generator, this will trigger assertion panic.