Add a way to use default field value as serde default

#![feature(default_field_values)]

#[derive(serde::Deserialize)]
struct S1 {
    tag: i8,
    data: Vec<i8> = Vec::new(),
}

#[derive(serde::Deserialize)]
struct S2 {
    #[serde(default)]
    field1: S1 = S1 { tag: 2, .. },
}

This code fails with:

error[E0277]: the trait bound `S1: Default` is not satisfied
  --> core_lib/src/lib.rs:11:5
   |
11 | /     #[serde(default)]
12 | |     field1: S1 = S1 { tag: 2, .. },
   | |__________________________________^ the trait `Default` is not implemented for `S1`
   |
help: consider annotating `S1` with `#[derive(Default)]`
   |
 4 + #[derive(Default)]
 5 | struct S1 {
   |

Would be nice to have a way to make serde use S1 { tag: 2, .. } as default value for S2.field1, when that field is not set during deserialization. Personally I'd say this could become the usual behavior of #[serde(default)] (do not require field type to implement Default, take default value from the one specified in struct). This way priority of sources for default would be:

1. Explicitly set source in macro
2. Field default value
3. Default implementation of type stored in the field

If it's not possible, having alternate way of doing it (e.g. #[serde(field_default)]) works as well.