rustdoc: Add support for associated items even outside the impl itself

Previously, associated items would only be available for linking
directly on the `impl Trait for Type`. Now they can be used anywhere.

- Make `item` for resolve mandatory
- Refactor resolving associated items into a separate function
- Remove broken trait_item logic
- Don't skip the type namespace for associated items
- Only call `variant_field` for `TypeNS`
- Add test for associated items
- Use exhaustive matches instead of wildcards

Wildcards caused several bugs while implementing this.

Author: jyn514

src/librustdoc/clean/types.rs

@@ -20,7 +20,7 @@ use rustc_hir::lang_items;
 use rustc_hir::Mutability;
 use rustc_index::vec::IndexVec;
 use rustc_middle::middle::stability;
-use rustc_middle::ty::TyCtxt;
+use rustc_middle::ty::{AssocKind, TyCtxt};
 use rustc_span::hygiene::MacroKind;
 use rustc_span::source_map::DUMMY_SP;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
@@ -288,6 +288,15 @@ impl ItemEnum {
             _ => false,
         }
     }
+
+    pub fn as_assoc_kind(&self) -> Option<AssocKind> {
+        match *self {
+            ItemEnum::AssocConstItem(..) => Some(AssocKind::Const),
+            ItemEnum::AssocTypeItem(..) => Some(AssocKind::Type),
+            ItemEnum::TyMethodItem(..) | ItemEnum::MethodItem(..) => Some(AssocKind::Fn),
+            _ => None,
+        }
+    }
 }
 
 #[derive(Clone, Debug)]