rustdoc: Cleanup associated const value rendering

Rather than (ab)using Debug for outputting the type in plain text use the
alternate format parameter which already does exactly that. This fixes
type parameters for example which would output raw HTML.

Also cleans up adding parens around references to trait objects.

src/librustdoc/html/render.rs

@@ -1662,9 +1662,9 @@ fn md_render_assoc_item(item: &clean::Item) -> String {
     match item.inner {
         clean::AssociatedConstItem(ref ty, ref default) => {
             if let Some(default) = default.as_ref() {
-                format!("```\n{}: {:?} = {}\n```\n\n", item.name.as_ref().unwrap(), ty, default)
+                format!("```\n{}: {:#} = {}\n```\n\n", item.name.as_ref().unwrap(), ty, default)
             } else {
-                format!("```\n{}: {:?}\n```\n\n", item.name.as_ref().unwrap(), ty)
+                format!("```\n{}: {:#}\n```\n\n", item.name.as_ref().unwrap(), ty)
             }
         }
         _ => String::new(),