Plumb returning event data from DOM context

[sjmiles]

Instead of sending only the handler name back, plumb the ports to handle an event(let) object, which contains the handler name and the target node's value (property) and key value. Most input elements use value for user-entered data, and key will be private data. For now, encode keys into the rendering stream (maybe attribute arc-key or data-arc-key), although if this is a sanitization issue, we can map key data to internally-generated ids for rendering.

See also #92

[sjmiles]

Completed by #105