rustdoc-search: add support for associated types

tests/rustdoc-js/gat.rs

@@ -0,0 +1,8 @@
+pub trait Foo {
+    type Assoc<T>;
+}
+
+pub fn sample<X: Foo<Assoc<u8> = u8>>(_: X) -> u32 { loop {} }
+pub fn synergy(_: impl Foo<Assoc<u8> = u8>) -> ! { loop {} }
+pub fn consider(_: impl Foo<Assoc<u8> = u32>) -> bool { loop {} }
+pub fn integrate<T>(_: impl Foo<Assoc<T> = T>) -> T { loop {} }