Merge sequence of async streams into single async stream

[atifaziz]

This PR adds Merge that concurrently merges all the elements of multiple asynchronous streams into a single asynchronous stream.

An overload allows concurrency to be controlled from 1 (sequential) to some N (or maximum degree of concurrency).

When concurrency is greater than 1, elements in the resulting stream may appear out of order.

If iterations are synchronous in whole or in part then the whole or parts are yielded/streamed synchronously without causing allocations.

This PR supersedes #775.

[viceroypenguin]

Should there also be a version with an explicit CancellationToken for those who use fluent syntax instead of foreach for processing IAsyncEnumerable<>s?

[atifaziz]

Should there also be a version with an explicit CancellationToken for those who use fluent syntax instead of foreach for processing IAsyncEnumerable<>s?

Most probably, but this is still in draft. 😉 However, the motivation for that isn't so much to satisfy the fluent syntax members of the audience but to allow options to be changed at time of composition or evaluation.

[viceroypenguin]

Completed in viceroypenguin/SuperLinq#35 - to be released in SuperLinq 4.1.0

[codecov]

Codecov Report

Merging #793 (28996d4) into master (4a9041d) will not change coverage.
The diff coverage is n/a.

❗ Current head 28996d4 differs from pull request most recent head 0fb644d. Consider uploading reports for the commit 0fb644d to get more accurate results

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