This whole thing changes it so that the JS and the UI both use
rustc's own path printing to handle the impl IDs. This results in
the format changing a little bit; full paths are used in spots
where they aren't strictly necessary, and the path sometimes uses
generics where the old system used the trait's own name, but it
shouldn't matter since the orphan rules will prevent it anyway.
Since these elements now use `white-space: pre-wrap` since
784665d4ce, it's fine to use newlines
for formatting, which is smaller and a bit less complicated.
This class originated in the very first commit of `rustdoc_ng`, and was used
to add a color border around the item decl based on its kind.
4fd061c426/src/rustdoc_ng/html/static/main.css (L102-L106)
The item decl no longer has a border, and there aren't any
kind-specific styles in modern rustdoc's rendering of this UI item.
Most of this commit is updating test cases so that they use `item-decl` to
find the `<pre>` tag instead of relying on the fact that the class name
had `rust {kind}` in it while other `<pre>` tags only had class `rust`.
