Hi, thanks for this awesome library! I have some questions regarding in-place deserialization.

I'm wondering why we are filling skipped fields in deserialize_in_place(). Skipped fields are ignored for maps but not for sequences. Is there a reason for it or should the filling with default be removed? This is the relevant code that writes default values into the in-place struct:

My use case is the following: I have a struct (MixedContent below) which contains fields that I want to serialize and fields that cannot be serialized and also don't implement Default. Something like this:

// serde_derive with Feature deserialize_in_place
#[derive(Deserialize, Serialize)]
struct MixedContent {
    #[serde(skip, default = "panic")]
    non_serializable: NonSerializable,

    serializable: Serializable,
}

fn panic() -> NonSerializable {
    panic!("deserialize not implemented")
}

struct NonSerializable {
    // ...
}

#[derive(Deserialize, Serialize)]
struct Serializable {
    // ...
}

Since I cannot implement Default for the NonSerializable but for Serializable, my idea was to use the deserialize_in_place functionality with skipped fields. With that I can create a custom constructor with more fancy logic and then use deserialize_in_place() afterwards on the struct. I think, using some DeserializeSeed is too complicated because it seems I need to implement it manually. Also custom home-grown derive macros doesn't feel right to achieve my scenario.

What I need to adjust from basic serde usage is

1. Generate a proper deserialize_in_place: Easy, just enable the feature flag
2. Disable the normal deserialize: Can be done via a serde(default = ...) attribute which never returns anything but fulfills the trait and makes the compiler happy
3. Deactivate all struct field writes of skipped fields: Doesn't work because the sequence code generation still contains them, see above

I'm willing to create a PR to remove the skipped field filling in sequences but wanted to make sure, nothing else gets broken by it (Chesterton's Fence). Do you have suggestions?