rustdoc-search: add support for associated types

tests/rustdoc-js/assoc-type.rs

@@ -0,0 +1,12 @@
+pub fn my_fn<X: Iterator<Item = Something>>(_x: X) -> u32 {
+    3
+}
+
+pub struct Something;
+
+pub mod my {
+    pub trait Iterator<T> {}
+    pub fn other_fn<X: Iterator<crate::Something>>(_: X) -> u32 {
+        3
+    }
+}