Rollup merge of #102680 - dtolnay:btreesend, r=thomcc

Fix overconstrained Send impls in btree internals

Fixes https://github.com/dtolnay/async-trait/issues/215.

Minimal repro:

```rust
use std::collections::btree_map::Iter;

fn require_send<T: Send>(_: T) {}

fn main() {
    require_send(async {
        let _iter = None::<Iter<(), &()>>;
        async {}.await;
    });
}
```

```console
error: higher-ranked lifetime error
 --> src/main.rs:6:5
  |
6 | /     require_send(async {
7 | |         let _iter = None::<Iter<(), &()>>;
8 | |         async {}.await;
9 | |     });
  | |______^
  |
  = note: could not prove `impl Future<Output = ()>: Send`
```

Not-quite-so-minimal repro:

```rust
use std::collections::BTreeMap;
use std::future::Future;

fn spawn<T: Future + Send>(_: T) {}

async fn f() {
    let map = BTreeMap::<u32, Box<dyn Send + Sync>>::new();
    for _ in &map {
        async {}.await;
    }
}

fn main() {
    spawn(f());
}
```

```console
error: higher-ranked lifetime error
  --> src/main.rs:14:5
   |
14 |     spawn(f());
   |     ^^^^^^^^^^
   |
   = note: could not prove `impl Future<Output = ()>: Send`
```

I am not familiar with the btree internals, but it seems clear to me that the `async fn f` above should return a Send future. Using HashMap instead of BTreeMap in that code makes it already return a Send future.

The _"higher-ranked lifetime error"_ message may be a regression in Rust 1.63. Using older compilers the error message was more detailed:

```console
error: implementation of `Send` is not general enough
  --> src/main.rs:14:5
   |
14 |     spawn(f());
   |     ^^^^^ implementation of `Send` is not general enough
   |
   = note: `Send` would have to be implemented for the type `alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Immut<'0>, u32, Box<(dyn Send + Sync + '1)>, alloc::collections::btree::node::marker::LeafOrInternal>`, for any two lifetimes `'0` and `'1`...
   = note: ...but `Send` is actually implemented for the type `alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Immut<'2>, u32, Box<dyn Send + Sync>, alloc::collections::btree::node::marker::LeafOrInternal>`, for some specific lifetime `'2`

error: implementation of `Send` is not general enough
  --> src/main.rs:14:5
   |
14 |     spawn(f());
   |     ^^^^^ implementation of `Send` is not general enough
   |
   = note: `Send` would have to be implemented for the type `alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Immut<'0>, u32, Box<(dyn Send + Sync + '1)>, alloc::collections::btree::node::marker::Leaf>`, for any two lifetimes `'0` and `'1`...
   = note: ...but `Send` is actually implemented for the type `alloc::collections::btree::node::NodeRef<alloc::collections::btree::node::marker::Immut<'2>, u32, Box<dyn Send + Sync>, alloc::collections::btree::node::marker::Leaf>`, for some specific lifetime `'2`
```

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