rustdoc-search: add slices and arrays to index

This indexes them as primitives with generics, so `slice<u32>` is
how you search for `[u32]`, and `array<u32>` for `[u32; 1]`.
A future commit will desugar the square bracket syntax to search
both arrays and slices at once.

tests/rustdoc-js/slice-array.js

@@ -0,0 +1,65 @@
+// exact-check
+
+const QUERY = [
+    'R<primitive:slice<P>>',
+    'primitive:slice<R<P>>',
+    'R<primitive:slice<Q>>',
+    'primitive:slice<R<Q>>',
+    'R<primitive:array<Q>>',
+    'primitive:array<R<Q>>',
+    'primitive:array<TraitCat>',
+    'primitive:array<TraitDog>',
+];
+
+const EXPECTED = [
+    {
+        // R<primitive:slice<P>>
+        'returned': [],
+        'in_args': [
+            { 'path': 'slice_array', 'name': 'alpha' },
+        ],
+    },
+    {
+        // primitive:slice<R<P>>
+        'returned': [
+            { 'path': 'slice_array', 'name': 'alef' },
+        ],
+        'in_args': [],
+    },
+    {
+        // R<primitive:slice<Q>>
+        'returned': [],
+        'in_args': [],
+    },
+    {
+        // primitive:slice<R<Q>>
+        'returned': [],
+        'in_args': [],
+    },
+    {
+        // R<primitive:array<Q>>
+        'returned': [
+            { 'path': 'slice_array', 'name': 'bet' },
+        ],
+        'in_args': [],
+    },
+    {
+        // primitive:array<R<Q>>
+        'returned': [],
+        'in_args': [
+            { 'path': 'slice_array', 'name': 'beta' },
+        ],
+    },
+    {
+        // primitive::array<TraitCat>
+        'in_args': [
+            { 'path': 'slice_array', 'name': 'gamma' },
+        ],
+    },
+    {
+        // primitive::array<TraitDog>
+        'in_args': [
+            { 'path': 'slice_array', 'name': 'gamma' },
+        ],
+    },
+];