Move to static members and overloads, to support cancellation token passing, `ValueTask` and `Async` in the module members

[abelbraaksma]

It is currently quite a pain to implement cancellation passing explicitly. As has been said in several issues already, we need to support this on each member to do it properly. Plus we will need overloads:

• Each method taking a TaskSeq should also seamlessly take a ConfiguredCancelableAsyncEnumerable<_>, which is available in .NET Standard 2.1. This would solve Support ConfiguredCancelableAsyncEnumerable<'T> #167, reported by @xperiandri (PS: to prevent the surface area to blow up, I will check if ofCancelable/toCancelable functions would serve the same purpose).
• Each method currently with the Async prefix should be able to take a Task (as they can, currently), an Async and a ValueTask. Writing functions like iterAsync, iterWithTask, iterWithValueTask etc is going to be too painful long-run.
• Each method should have an optional cancellation token argument as their last argument (?), so that piping into the functions is still possible, but it will default to no cancellation token. Only members that consume the task sequence should get a cancellation token argument.

[abelbraaksma]

Some more original discussion can be found here: #133.

[abelbraaksma]

This change has meanwhile been merged into main.