Make our `Node`/`Element`/etc. hierarchy immutable.

The types surfaced by this framework should be immutable (`@dataclass(frozen=True)`).

We should encourage people to write functions (including component functions and, if we get around to it, possibly middleware) to alter these items.

We _should_ provide convenience routines for common operations (`Element.append()` and `Element.extend()`, for instance) that return new instances.