In the old setup, if the dereffed-to item has multiple impl blocks,
each one gets its own `div.impl-items` in the section, but there
are no headers separating them. Since the last method in a
`div.impl-items` has no bottom margin, and there are no margins
between these divs, there is no margin between the last method
of one impl and the first method of the following impl.
This patch fixes it by simplifying the HTML. Each Deref block gets
exactly one `div.impl-items`, no matter how many impl blocks it
actually has.
