Multiple bindings in a single event handler

[Rich-Harris]

Ref #910

[Rich-Harris]

This requires a bit of refactoring — checked and indeterminate should both be inspected in the same change event handler, but at the moment element bindings are dealt with one at a time, making that impossible.

[Rich-Harris]

This is quite a substantial PR, and I'm not completely happy with it — there's still lots of awkward special cases. But I think it's a bit nicer than what we had before, and more importantly it allows us to deal with multiple bindings in a single handler. So the change event on an <input type='checkbox'> updates both checked and indeterminate, and likewise for media elements with currentTime and played bindings, or buffered and seekable.

It also tidies a couple of other things up, such as moving some <select> handling into helper functions.

I think it's probably mergeable, though there might be some remaining opportunities to neaten things up a bit.