Re: [w3c/webcomponents] Need "slotchange" event (#288)

Here is some pseudo-code I plan to adopt for the DOM Standard next week. Hopefully folks can take a look at it and hopefully it's understandable. I'm assuming that slotables have pointers to slots and slots hold a list of pointers to slotables and both need updating during mutations. Flattening is not calculated as that does not seem necessary. CSS and the `assignedNodes()` method can handle flattening by themselves.
```
insert

  The node is being inserted into a parent node that also has a shadow root
    -> let slot be findSlot(node)
    -> if slot is non-null:
      -> run assignSlotables(slot) // this is naive, but good enough for spec
      -> slotchange(slot)

  The node is being inserted into a parent node that is a slot
    -> assignSlotables(parent)
    -> if node's assigned slot is non-null, slotchange(node's assigned slot)

  An inclusive-descendant of node is a slot element
    -> for each _slot_ in node's tree // a new slot changes everything
      -> let current be _slot_'s assigned nodes
      -> assignSlotables(_slot)
      -> let new be _slot_'s assigned nodes
      -> if current != new and _slot_ was not just inserted, slotchange(_slot_)

remove

  Node has an assigned slot
    -> slotchange(node's assigned slot) // recursive
    -> run assignSlotables(node's assigned slot)
    -> Assert: node's assigned slot is null

  Node has a slot element as inclusive-descendant
    -> for each _slot_ in node's tree:
      -> let current be _slot_'s asigned nodes
      -> assignSlotables(_slot_)
      -> let new be _slot_'s assigned nodes
      -> if current != new, slotchange(_slot_)
    -> for each _slot_ in oldParent's tree // spec calls this parent
      -> let current be _slot_'s asigned nodes
      -> assignSlotables(_slot_)
      -> let new be _slot_'s assigned nodes
      -> if current != new, slotchange(_slot_)

slotable's slot attribute change
  -> if value == oldValue, return
  -> if node's assigned slot is non-null:
    -> assignSlotables(node's assigned slot)
    -> slotchange(node's assigned slot)
  -> Assert: node's assigned slot == null
  -> let slot be findSlot(node)
  -> if slot is non-null
    -> assignSlotables(slot)
    -> slotchange(slot)

slot's name attribute change
  -> for each slot element _slot_ in slot element's tree
    -> let current be _slot_'s asigned nodes
    -> assignSlotables(_slot)
    -> let new be _slot_'s assigned nodes
    -> if current != new, slotchange(_slot_)
```
Obviously there's a bit of copypasta here that can be (and will be in the spec) reduced with extra algorithms.

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