rustdoc: add support for type filters in arguments and generics

This makes sense, since the search index has the information in it,
and it's more useful for function signature searches since a
function signature search's item type is, by definition, some type
of function (there's more than one, but not very many).

tests/rustdoc-js/generics.js

@@ -2,6 +2,8 @@
 
 const QUERY = [
     'R<P>',
+    'R<struct:P>',
+    'R<enum:P>',
     '"P"',
     'P',
     'ExtraCreditStructMulti<ExtraCreditInnerMulti, ExtraCreditInnerMulti>',
@@ -20,6 +22,20 @@ const EXPECTED = [
             { 'path': 'generics', 'name': 'alpha' },
         ],
     },
+    {
+        // R<struct:P>
+        'returned': [
+            { 'path': 'generics', 'name': 'alef' },
+        ],
+        'in_args': [
+            { 'path': 'generics', 'name': 'alpha' },
+        ],
+    },
+    {
+        // R<enum:P>
+        'returned': [],
+        'in_args': [],
+    },
     {
         // "P"
         'others': [