Immovable generators and unsafe block

[rozaliev]

Immovable generators require unsafe block, thus allowing every unsafe function inside generator to work. There is no way to say I want immovable generator, but its body should not be marked unsafe.

unsafe { 
    static || {
        yield;
        let a =  Box::from_raw(0 as *mut usize);
    }
};

It can probably be fixed by adding new ImmovableGenerator trait and making creation of immovable generators safe.

cc @Zoxc