Add par_map methods to collections

[brendanmaguire]

• par_map facilitates running an async function across all items in the collection
• Resolves Async methods #213

[brendanmaguire]

@dbrattli , do you think this is an okay approach to this?

[timok19]

HI! Is there any updates on that? :)

[dbrattli]

Thanks for the async collection proposal. Before this can be reviewed, please rebase it onto current main (the branch is 27 commits behind) and address these blockers:

1. Map.par_map consumes self.to_seq() while constructing the coroutines, then consumes it again for keys. The Map iterator is single-use, so the second pass is empty and a successful call returns an empty map. Materialize the item pairs once (or carry each key through the gathered work) and add a regression assertion that the result contains all original keys.
2. The tests assert a < 0.2 wall-clock duration for 0.1-second sleeps. That is scheduler/load dependent and will be flaky in CI. Test concurrent start/completion deterministically (for example, with synchronization primitives) instead of elapsed time.

Please retain ordering and error-propagation coverage across all collection variants when rebasing.

[brendanmaguire]

I won't have time to work on this. If someone else wants to pick it up, feel free. Or close if you'd like.