This implements manual slottable assignment by way of HTMLSlotElement's
`assign` API. This includes all of the slottable-related AOs needed to
perform the assignment.
A slottable is either a DOM element or a DOM text node. They may be
assigned to slots (HTMLSlotElement) either automatically or manually.
Automatic assignment occurs by matching a slot's `name` attribute to
a slottable's `slot` attribute. Manual assignment occurs by using the
slot's (not yet implemented) `assign` API.
This commit does not perform the above assignments. It just sets up the
slottable concept via IDL and hooks the slottable mixin into the element
and text nodes.
