Make it possible to map listeners to a different type

[cramertj]

Equivalent to Elm's map. I've been working on it in the mappable-listeners branch, but it's extremely messy and requires a bunch of extra traits. A much simpler solution would be possible with ATCs on types (not just lifetimes), as then DomNode, DomNodes, Listener, and Listeners could all implement some sort of MessageFunctor trait.

[cramertj]

Here's an example of how one could create a mappable component without this feature:

struct ComponentMsg;
fn component_with_mappable_msg<'a, Mapper, Msg>(msg_mapper: Mapper)
    -> impl DomNode<Message = Msg> + 'a
    where
    Msg: 'a,
    Mapper: Fn(ComponentMsg) -> Msg + 'a
{
    button((
        on("click", move |_| msg_mapper(ComponentMsg))
    ))
}