Add a `defer` attribute

Some `KV` types may contain other `KV` types, it'd be nice if we invoked `KV::serialize()` on those instead of `Value::serialize()`.

e.g.

```rust
#[derive(KV)]
pub struct Foo {
  #[slog(defer)]
  bar: Bar,
}

#[derive(KV)]
pub stuct Bar {
  x: usize,
  path: String.
}
```