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

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 `AsyncSequence`s that have the same element type to cover these use cases.

Start by reviewing the existing combineLatest, zip, and merge implementations and their variadic APIs. Define how Sequence-based inputs of same-element-type AsyncSequences should behave, then verify that the resulting APIs cover runtime-sized collections of sequences and match the existing algorithm semantics.

Written by the indexing model from the issue text.