Assert that DOM structs have the correct first field

DOM structs embed their parent type as their first field. This
introduces a `.parent()` method to the DOM struct that returns its first
field, and codegens a type assert that ensures that `.parent()` returns
the parent struct.

This generates:

On `#[dom_struct]`:

```rust
impl HasParent for Type {
    type Parent = ParentType;
    fn as_parent(&self) -> ParentType {
        &self.first_field
    }
}
```

In the codegen files:

```rust
impl Type {
    fn __assert_parent_type(&self) {
        let _: &ParentType = self.as_parent();
    }
}
````

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