pretty print BTreeSet
I want pretty printing for BTreeSet.
```rust
use std::collections::*;
fn main() {
let mut s = BTreeSet::new();
s.insert(5);
s.insert(3);
s.insert(7);
s.remove(&3);
println!("{:?}", s);
}
```
```
(gdb) b 9
(gdb) p s
$1 = BTreeSet<i32> with 2 elements = {[0] = 5, [1] = 7}
```
This is analogy of pretty printing for C++ std::set.
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