Extract Tree api into deref target

[molpopgen]

Work going on over in #296 risks code duplication.
To eliminate this, we need to separate out the public
API of a tree from its storage requirements.

Roughly, something like this should do the trick:

// Gets the public API
struct TreeApi {
    tree: NonNull<ll_bindings::tsk_tree_t>,
}

// Worries about storage, Drop, etc..
struct TreeTest {
    tree: ll_bindings::tsk_tree_t,
    api: TreeApi,
}

impl TreeTest {
    fn new(ts: TreeSequence) -> Self {
        let mut m = MaybeUninit::<ll_bindings::tsk_tree_t>::uninit();
        let rv = unsafe { ll_bindings::tsk_tree_init(m.as_mut_ptr(), ts.as_ptr(), 0) };
        assert_eq!(rv, 0);
        let mut tree = unsafe { m.assume_init() };
        let tp = &mut tree as *mut ll_bindings::tsk_tree_t;

        Self {
            tree,
            api: TreeApi {
                tree: unsafe { NonNull::new_unchecked(tp) },
            },
        }
    }
}

Once done, we can have separate owning/non-owning Tree types.

[molpopgen]

Closed by #300