* [Observation] Exclude generic class types from pre-caching their keypaths
* Account for multiple nesting levels of generics for observation cached keypaths
* [Observation] Forward availability and defines to extensions
* Simplify availability slightly from review feedback
* Simplify availability for extensions to use `.with`
With support for redundant conformance declarations via macros,
the `Observable` protocol can be a non-marker protocol, which
provides more flexibility for evolution in the future.
rdar://111463883
This change also switches to the new ExtensionMacro protocol,
the requirement for which includes information about whether the
conformance to the Observable protocol has already been added, either
in the declaration or in a superclass to the macro-attributed type.
This allows the @Observable macro to be applied to subclasses of
observable types without redundant-conformance errors.
* Make ObservationRegistrar Codable/Hashable
These conformances enable automatic Codable synthesis for Observable
types, and smooth the runway for structs being supported by the
Observable macro in the future.
* Limit Observable macro to classes
This removes the ability for the Observable macro to apply to structs,
and adds diagnostic tests for the three disallowed declaration kinds.
