Option<Box<T<'a>>> fields where Talso has into_owned

[ids1024]

I'm trying to use this for the SinkInfo<'a> in libpulse_binding (jnqnfe/pulse-binding-rust#44), but it has a field active_port: Option<Box<SinkPortInfo<'a>>>.

I guess to handle this, it would need to have logic to handle Boxes, like it handles Option, and FieldKind::resolve would have to be recursive to allow a Box inside an Option. I'm looking into how to implement this, since recursing in particular changes some things.

I guess it could also be helpful with complicated structs to have something like serde's deserialize_with to provide a custom into_owned for a field...