Add functions to partition the email into parts

It would be useful to be able to partition emails into mutually exclusive collectively exhaustive (MECE) parts.
We are already half way there with the functions

```rust
Message::attachments()
Message::html_bodies()
Message::text_bodies()
```

but as far as I can see, there is nothing like

```rust
Message::other_parts()
```

that will list all parts that are not attachments or bodies.