Add support for borrowed deserialization

[TedDriggs]

I have a struct like this:

#[derive(Deserialize)]
struct Example<'a> {
    #[serde(borrow)]
    field: Option<Cow<'a, str>>,
    #[serde(borrow, default)]
    other_field: Vec<Cow<'a, str>>,
}

I'd like to use it with this crate, but the DeserializeOwned bound gets in the way.

I think the issue is that deserialization goes through Document, which in turn requires ownership of strings etc. inside it. Is that correct, and if so could that be mitigated by having a bson::borrow::Bson which points into a backing &'a [u8]?

[pooooodles]

Tracked in https://jira.mongodb.org/browse/RUST-688

[pooooodles]

Hi Ted, just wanted to follow up to let you know this is still on our radar. We'll be taking a deeper look after we wrap up the work and conversation on #229

[patrickfreed]

This is implemented in 7ccf82b and will be included in the next release.