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;
  - it uses `.ready()?` since it's such a neat pattern;
  - it renames `Took` to `Taken` for consistency with `Done`.

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