Rollup merge of #91721 - danielhenrymantilla:patch-1, r=joshtriplett

Minor improvements to `future::join!`'s implementation

This is a follow-up from #91645, regarding [some remarks I made](https://rust-lang.zulipchat.com/#narrow/stream/187312-wg-async-foundations/topic/join!/near/264293660).

Mainly:
  - it hides the recursive munching through a private `macro`, to avoid leaking such details (a corollary is getting rid of the need to use ``@`` to disambiguate);
  - it uses a `match` binding, _outside_ the `async move` block, to better match the semantics from function-like syntax;
  - it pre-pins the future before calling into `poll_fn`, since `poll_fn`, alone, cannot guarantee that its capture does not move (to clarify: I believe the previous code was sound, thanks to the outer layer of `async`. But I find it clearer / more robust to refactorings this way 🙂).
  - it uses `@ibraheemdev's` very neat `.ready()?`;
  - it renames `Took` to `Taken` for consistency with `Done` (tiny nit 😄).

~~TODO~~Done:

  - [x] Add unit tests to enforce the function-like `:value` semantics are respected.

r? `@nrc`

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