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/tuple-unit.js

@@ -57,7 +57,7 @@ const EXPECTED = [
         'in_args': [],
     },
     {
-        'query': '(Q, ())',
+        'query': '(Q, R<()>)',
         'returned': [
             { 'path': 'tuple_unit', 'name': 'nest' },
         ],
@@ -71,7 +71,7 @@ const EXPECTED = [
         'in_args': [],
     },
     {
-        'query': '(u32)',
+        'query': 'R<(u32)>',
         'returned': [
             { 'path': 'tuple_unit', 'name': 'nest' },
         ],