Fix potential deadlocks when resuming a continuation while holding a …

…lock (apple#303)

* Fix potential deadlocks in zip

* Fix debounce

* Fix combineLatest

* Fix Channel

* Fix buffer

Make throttle underscored (apple#296)

# Motivation
During the discussion in apple#248 it became clear that the semantics of `throttle` are not 100% figured out yet.

# Modification
This PR is making `throttle` an underscored API for the upcoming 1.0.0 release. This gives us more time to investigate what exact semantics we want to have.

Rework availability for executor to avoid warnings (apple#288)

buffer: implement with custom storages (apple#239)

AsyncLazySequence proposal (apple#189)

Remove the disable of availability and mark methods as available to c…

…lock/instant/duration (apple#165)

* Remove the disable of availability and mark methods as available to clock/instant/duration

* Rework availability to be more permissive for tests

* Use hand coded values instead of defines (only available via Darwin versions of Dispatch)

* Adjust additional test availability

* Correct debounce test availability guard

* Update build instructions to indicate Xcode requirements

* Ensure the clock property of the validation diagram is only available for targets that can support it

Update availability to be in sync with Clock/Instant/Duration (apple#159

)

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