rustdoc-search: simplify rules for generics and type params

This commit is a response to feedback on the displayed type
signatures results, by making generics act stricter.

Generics are tightened by making order significant. This means
`Vec<Allocator>` now matches only with a true vector of allocators,
instead of matching the second type param. It also makes unboxing
within generics stricter, so `Result<A, B>` only matches if `B`
is in the error type and `A` is in the success type. The top level
of the function search is unaffected.

Find the discussion on:

* <449965149>
* <https://github.com/rust-lang/rust/pull/124544#issuecomment-2204272265>
* <476841363>

tests/rustdoc-js/nested-unboxed.js

@@ -33,9 +33,8 @@ const EXPECTED = [
     },
     {
         'query': '-> Result<i32, u32, bool>',
-        'others': [
-            { 'path': 'nested_unboxed', 'name': 'something' },
-        ],
+        // can't put nested generics out of order
+        'others': [],
     },
     {
         'query': '-> Result<Object<i32>, bool>',
@@ -45,9 +44,7 @@ const EXPECTED = [
     },
     {
         'query': '-> Result<Object<u32>, bool>',
-        'others': [
-            { 'path': 'nested_unboxed', 'name': 'something' },
-        ],
+        'others': [],
     },
     {
         'query': '-> Result<Object<i32>, u32, bool>',