AsyncMap removal

This update is small and straightforward:

• asyncMap & asyncTryMap have been removed since they don't play nicely with Combine's backpressure mechanisms.
• Some minor tweaks and warnings have been fixed for Xcode 12.

Minor tweaks & performance optimizations

0.5.1

Turn Lock into a value type

Better Backpressure & Custom Subscribers

This release includes two new useful custom subscribers and a bunch of interesting operators:

• retry on delayed intervals.
• handleEnd to clean up resources in any completion case (i.e. successful/failure completion or cancellation).
• sink using the new custom subscribers.

Also, there is better support for backpressure all around the framework. Many operators/publishers now accept optional "demand" related parameters and work great with standard backpressure mechanisms (such as buffer).

Enhanced FlatMaps & Standard Naming For Deferred Publishers

The sequential flatMap operator/publisher has been re-engineered to accept a transform closure, making it much more useful.

Conbini For Testing

Conbini has been separated into two products:

• import Conbini for regular usage in your framework or application.
• import ConbiniForTesting including the testing conveniences (which depend on XCTest).

First Release

Initial release gathering the first batch of publishers/subscribers.