Rollup merge of #53112 - fukatani:pretty-print-btreeset, r=michaelwoerister

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.

0 files changed, 0 insertions, 0 deletions