Request for Comments: Adding split

[aDifferentJT]

I've added an initial implementation of split that I'm using in my own project, and appears to work there. I'm aware that this PR is currently incomplete, but I'd like to know whether this is something people feel is worth adding here, and if so, if this is the right design, before building out the rest of the stuff (units tests etc).

There are currently two different split implementations, one returning an async sequence of collections and the other returning an async sequence of async sequences.

I can see value in both, and one of the questions that warrants this Request for Comments is which is preferred, or whether both should be included.

[phausler]

There are a few additional steps here we would likely want to do to go after this:

1. some examples and perhaps a pitch document would be ideal to explain the usage and utility
2. ideally this should have some unit tests to show that it works as expected
3. there would need to be some changes to the API to make it actually public instead of package internal (ideally it should return an opaque type from the AsyncSequence extension functions).

generally i do wonder if it needs the buffering of the AsyncStream underpinning it, could it perhaps use flatMap + reduce to practically achieve the same thing?