rustdoc: fix weird margins between Deref impl items
## Before
## After
## Description
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.
39ed894926
