rustdoc-search: build args, return, and generics on one unifier

This enhances generics with the "unboxing" behavior where A<T>
matches T. It makes this unboxing transitive over generics.

tests/rustdoc-js/nested-unboxed.rs

@@ -0,0 +1,5 @@
+pub struct Object<T, U>(T, U);
+
+pub fn something() -> Result<Object<i32, u32>, bool> {
+    loop {}
+}