`Sequence`-based implementation of combining algorithms in addition to variadic ones

[MaxDesiatov]

Existing implementations of combineLatest , zip , merge are great when the total number of sequences is known at compile time (2 or 3 without variadic generics). In some scenarios, these sequences have same element types, but the total number of sequences is computed at run time. Existing implementations won't work here, and I think the library would benefit from implementations that take a Sequence (or some other container protocol) of AsyncSequences that have the same element type to cover these use cases.

[FranzBusch]

Thanks for opening this @MaxDesiatov. This is something we can look into after the 1.0 release!

[clive819]

This is my attempt to make combineLatest support variadic parameters.

https://github.com/apple/swift-async-algorithms/pull/322/files