letdom and dom-node does not work (as described)

The documentation of `letdom` is quite outdated, but in particular states that one could access 'bound' dom element in its 'own' event handler:

```
(reacl.core/defclass search-bar
app-state [filter-text in-stock-only on-user-input]
render
(fn [& {:keys [dom-node]}]
(dom/letdom
[textbox (dom/input
{:type \"text\"
:placeholder \"Search...\"
:value filter-text
:onChange (fn [e]
(on-user-input
(.-value (dom-node textbox))
(.-checked (dom-node checkbox))))})
checkbox (dom/input
{:type \"checkbox\"
:value in-stock-only
:onChange (fn [e]
(on-user-input
(.-value (dom-node textbox))
(.-checked (dom-node checkbox))))})]
(dom/form
textbox
(dom/p
checkbox
\"Only show products in stock\")))))
```

1. render as a function of `{:dom-node ..}` is not possible anymore, I'm quite sure.
2. a global `dom-node` function exists, but that takes the component as a first arg.
3. Calling `dom-node` in an event handler like here returns `undefined` (at least in a simplified test-case)
What does still work is rendering the things bound by `letdom`.

Given that `refs` have been introduced a while ago, I'd say we can quickly phase out `letdom` all together. Also, people collect the states of the two components in a common local or app-state anyway, and never access anything else than the `.-target` of an event handler.