Why is DateTime 36 bytes?

```rust
std::mem::size_of::<jiff::civil::DateTime>() = 36;
std::mem::size_of::<chrono::DateTime<chrono::FixedOffset>>() = 16;
```

in comparison `chrono::DateTime` is just 16 bytes.

```rust
std::mem::size_of::<jiff::Timestamp>() = 40;
std::mem::size_of::<jiff::Zoned>() = 96;
```

Timestamp is also quite big for what can be represented with u128, but Zoned is even bigger.

Why is that?