rustdoc: implement bag semantics for function parameter search

This tweak to the function signature search engine makes things so that,
if a type is repeated in the search query, it'll only match if the
function actually includes it that many times.

tests/rustdoc-js/search-bag-semantics.js

@@ -0,0 +1,20 @@
+// exact-check
+
+const QUERY = [
+    'P',
+    'P, P',
+];
+
+const EXPECTED = [
+    {
+        'in_args': [
+            { 'path': 'search_bag_semantics', 'name': 'alacazam' },
+            { 'path': 'search_bag_semantics', 'name': 'abracadabra' },
+        ],
+    },
+    {
+        'others': [
+            { 'path': 'search_bag_semantics', 'name': 'abracadabra' },
+        ],
+    },
+];