Fix: remove overlapping operators from swift-async-algorithms

[lachenmayer]

Description

Hi there, I'm currently moving our codebase from Combine to Swift concurrency – loving this library, it has a bunch of really excellent operators and helpers.

I bumped into an issue in our project where I couldn't use the [1, 2, 3].async extension because this is also implemented in swift-async-algorithms. There seems to be no way for me to fix this in downstream code, because I can't selectively import extensions (from what I can tell!).

Now that swift-async-algorithms has hit 1.0, I feel like this could be a good time to remove the overlapping operators.

This PR removes:

• AsyncZip2Sequence, AsyncZip3Sequence
• AsyncMerge2Sequence, AsyncMerge3Sequence, but not the variadic AsyncMergeSequence, as this does not exist in swift-async-algorithms (yet?)
• AsyncLazySequence (this is replaced by the now renamed AsyncSyncSequence in swift-async-algorithms)

Checklist

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

I did not update the CHANGELOG, as I'm not sure what version this should be (possibly v0.6.0?). Happy to do that if you like.