## flat_tree
### Instructions
Create the `flatten_tree` **function** which receives a `std::collections::BTreeSet` and returns a new `Vec` with the elements of the binary tree in order.
### Expected function
```rust
pub fn flatten_tree< T: ToOwned < Owned = T > >(tree: & BTreeSet< T > ) -> Vec< T > {
}
```
### Usage
Here is a possible program to test your function:
```rust
use flat_tree::*;
use std::collections::BTreeSet;
fn main() {
let mut tree = BTreeSet::new();
tree.insert(34);
tree.insert(0);
tree.insert(9);
tree.insert(30);
println!("{:?}", flatten_tree(&tree));
let mut tree = BTreeSet::new();
tree.insert("Slow");
tree.insert("kill");
tree.insert("will");
tree.insert("Horses");
println!("{:?}", flatten_tree(&tree));
}
```
And its output:
```console
$ cargo run
[0, 9, 30, 34]
["Horses", "Slow", "kill", "will"]
$
```
