project: make streams async (using actors) + improve concurrent iterators

[twittemb]

Description

make streams async (using actors) + improve concurrent iterators

Checklist

• this PR is based on the main branch and is up-to-date, if not please rebase your branch on the top of main
• the commits inside this PR have explicit commit messages
• unit tests cover the new feature or the bug fix
• the feature is documented in the README.md if it makes sense
• the CHANGELOG is up-to-date

[codecov-commenter]

Codecov Report

Merging #6 (ba369c0) into main (e4aeb0f) will decrease coverage by 0.16%.
The diff coverage is 97.93%.

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   97.82%   97.66%   -0.17%     
==========================================
  Files          40       42       +2     
  Lines        1887     2139     +252     
==========================================
+ Hits         1846     2089     +243     
- Misses         41       50       +9     

Impacted Files	Coverage Δ
Sources/Internal/AsyncIteratorByRef.swift	100.00% <ø> (ø)
...ts/Operators/AsyncSequence+HandleEventsTests.swift	89.47% <ø> (ø)
.../Operators/AsyncSequence+SwitchToLatestTests.swift	94.11% <66.66%> (-1.34%)	⬇️
Sources/Operators/AsyncSequence+HandleEvents.swift	97.77% <90.00%> (+6.66%)	⬆️
...ests/AsyncSequences/AsyncSequences+FailTests.swift	80.00% <90.00%> (+3.07%)	⬆️
Sources/AsyncSequences/AsyncSequences+Merge.swift	95.12% <92.72%> (-2.29%)	⬇️
...s/Operators/AsyncSequence+FlatMapLatestTests.swift	88.13% <93.33%> (-5.33%)	⬇️
Tests/Internal/SharedAsyncIteratorTests.swift	97.75% <97.75%> (ø)
Sources/AsyncSequences/AsyncSequences+From.swift	100.00% <100.00%> (ø)
Sources/AsyncSequences/AsyncSequences+Zip.swift	97.45% <100.00%> (ø)
... and 16 more

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