Provide a higher arity version of merge(…)

I've got a use case where I have an unknown number of `AsyncChannel`s that I want to merge into a single stream of events.

It'd be wonderful if `merge(…)` and friends supported more than an arity of 3 - ideally accepting a sequence?

Thanks!

Start by locating the existing merge(...) and related arity-three APIs, then read the tests that exercise them. Determine how a sequence of AsyncChannels should be represented and confirm the expected behavior for an unknown number of inputs. Done means the merge APIs support the requested higher-arity or sequence-based use case with test coverage.

Written by the indexing model from the issue text.