Interoperating with existing F# async

[dsyme]

We could allow binding to an existing F# Async, passing the cancellation token through.

Similar to this in the existing AsyncSeq: https://github.com/fsprojects/FSharp.Control.AsyncSeq/blob/main/src/FSharp.Control.AsyncSeq/AsyncSeq.fs#L425

[abelbraaksma]

See #114 for a first attempt at this.

[abelbraaksma]

This has now been implemented and will be part of the 0.4.0 release cycle.

Cancellation token passing in the general sense is still up for discussion, see #133, /cc @dsyme.