the `just` senders should have a parallel forward progress guarantee

[ericniebler]

just, just_stopped, and just_error do not do anything that would prevent them from making parallel forward progress.

Proposed resolution

Change the last paragraph of [exec.just]/p2 as follows:

 For just, just_error, and just_stopped, let set-cpo be set_value, set_error,
 and set_stopped, respectively. The exposition-only class template impls-for
 ([exec.snd.general]) is specialized for just-cpo as follows:
 
 namespace std::execution {
   template<>
   struct impls-for<decayed-typeof<just-cpo>> : default-impls {
+    static constexpr auto get-attrs =
+      [](auto&&) noexcept {
+        struct attrs { // exposition only
+          static constexpr forward_progress_guarantee query(get_forward_progress_guarantee_t) noexcept {
+            return forward_progress_guarantee::parallel;
+          }
+        };
+        return attrs{};
+      };
 
     static constexpr auto start =
       [](auto& state, auto& rcvr) noexcept -> void {
         auto& [...ts] = state;
         set-cpo(std::move(rcvr), std::move(ts)...);
       };
   };
 }