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();
}
}
````
