Fix crash when multiple updates create publishers simultaneously

Initial support for caching values in-memory

Fix deadlock when adding a new subscriber inside the closure of an update listener

Support animations when using Binding provided via PersistStorage.projectedValue

Increase deployment targets to fix archive builds using Xcode 13

Thread-safety has been improved, including:

• Adding and removing update listeners
• Creating default values
• Setting values on InMemoryStorage

Support for Dates in UserDefaultsStorage

Add Persister/Persisted extensions on watchOS

• Add support for builder(for:) function to aid with building complex type (thanks to @randomeizer)
• Support for Xcode 12.1

Improve API for Subscription/Cancellable

Add documentation

Much better handling of optional values, which also allows for non-optional values

Release candidate 1 for 1.0 stable release

Has lots of tests but little documentation. API has changes a lot since initial 0.1.0 but I’m happy with it now. Hopefully it is consistent and (once docs are added) easy to use.