`done()` is unsound and should be marked `unsafe`

[jonas-schievink]

done is equivalent to mem::uninitialized and thus can be used to trivially construct uninitialized instances of types.

The function is documented "don't use it directly, use done!() macro instead", but it's neither hidden nor marked as unsafe, both should probably be done.

This results in a segmentation fault on my machine:

use generator::{co_get_yield, co_set_para, Gn, done};

fn main() {
    done::<(String, Vec<u128>, Box<[u128]>)>();
}

[Xudong-Huang]

done can't not be called in non-coroutine context. Will add more checks here.

[Xudong-Huang]

done should be a safe API. in generator context, we didn't touch the uninit memory. and will panic in non-generator context.