Fix async channel finishes all (apple#152)

* asyncChannel: enforce termination for all when finished

* asyncThrowingChannel: enforce termination for all when finished

* asyncChannel: improve unit tests readability

* asyncChannel: adapt the Guide to the new finish behaviour

* asyncChannel: adapt the Guide to the new finish behaviour

Add shims and adjust availability to allow for swift 5.6 toolchains t…

…o build until source stability is achieved (apple#100)

* Add shims and adjust availability to allow for swift 5.6 toolchains to build until source stability is achieved

* Restore availability again