Doesn't build with rustc 1.73

```
error[E0446]: crate-private type `Conn<'a>` in public interface
--> src/rusttls/stream.rs:13:5
|
13 | pub conn: Conn<'a>,
| ^^^^^^^^^^^^^^^^^^ can't leak crate-private type
...
17 | pub(crate) enum Conn<'a> {
| ------------------------ `Conn<'a>` declared as crate-private

error[E0446]: crate-private type `Conn<'a>` in public interface
--> src/rusttls/stream.rs:127:5
|
17 | pub(crate) enum Conn<'a> {
| ------------------------ `Conn<'a>` declared as crate-private
...
127 | pub fn new(io: &'a mut IO, conn: impl Into>) -> Self {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't leak crate-private type
```

This is just `cargo +1.73 check` in this repository.

Start in src/rusttls/stream.rs at lines 13, 17, and 127, where cargo +1.73 reports crate-private types in public interfaces. Run cargo +1.73 check while investigating those declarations; done means the repository builds without these E0446 errors.

Written by the indexing model from the issue text.