Make Impl.trait_ a Path, not a Type

It should only ever be a `ResolvedPath`, so this (a) enforces that, and
(b) reduces the size of `Impl`.

I had to update a test because the order of the rendered auto trait impl
bounds changed. I think the order changed because rustdoc sorts auto
trait bounds using their `Debug` output.

src/librustdoc/html/render/mod.rs

@@ -2056,10 +2056,10 @@ fn sidebar_struct(cx: &Context<'_>, buf: &mut Buffer, it: &clean::Item, s: &clea
 
 fn get_id_for_impl_on_foreign_type(
     for_: &clean::Type,
-    trait_: &clean::Type,
+    trait_: &clean::Path,
     cx: &Context<'_>,
 ) -> String {
-    small_url_encode(format!("impl-{:#}-for-{:#}", trait_.print(cx), for_.print(cx),))
+    small_url_encode(format!("impl-{:#}-for-{:#}", trait_.print(cx), for_.print(cx)))
 }
 
 fn extract_for_impl_name(item: &clean::Item, cx: &Context<'_>) -> Option<(String, String)> {