Add initial attempt at implementing BitOr for BTreeSet.
Update the implementation of the bitor operator for BTreeSets.

`make check` ran fine through this.
Add implementations for BitAnd, BitXor, and Sub as well.
Remove the FIXME comment and add unstable flags.
auto merge of #19514 : jbranchaud/rust/add-btree-set-bitor, r=alexcrichton

I am trying to add an implementation of `bitor` for `BTreeSet`. I think I am most of the way there, but I am going to need some guidance to take it all the way.

When I run `make check`, I get:

```
error: cannot move out of dereference of `&`-pointer
self.union(_rhs).map(|&i| i).collect::<BTreeSet<T>>()
                      ^~
```

I'd appreciate any nudges in the right direction. If I can figure this one out, I am sure I will be able to implement `bitand`, `bitxor`, and `sub` as well.

/cc @Gankro 

---

**Update**

I have added implementations for `BitOr`, `BitAnd`, `BitXor`, and `Sub` for `BTreeSet`.