Eliminate the use of the type-erased `AnyActorIdentity` within the
distributed actor protocol by exposing the identity type of an actor
transport as an associated type, `Identity`, which is then used to
refer to all actors that have that transport.
Retain `AnyActorIdentity`, because it remains useful for type-erasing
actor transports, especially as the `Identity` type for
`AnyActorTransport`. For now, make it the default type of `Identity`:
this helps smooth over the transition from use of `AnyActorIdentity`,
but we might want to remove this to make use of the type-erased forms
always opt-in.
Eliminate the required use of existentials in distributed actors by
introducing the `Transport` associated type into the
`DistributedActor` protocol. Each distributed actor has a known
(concrete) actor transport type, reducing storage requirements and
transport dynamism when it isn't needed.
Distributed actors can manually specify their `Transport` associated
type or pick up a default by looking for a type named
`DefaultActorTransport`. A library that vends an actor transport can
make create a public typealias `DefaultActorTransport` referring to
its transport, so importing that library and defining a distributed
actor will use that library's transport.
Introduce a type-erased `AnyActorTransport` type to provide an
explicitly dynamic actor transport. This is still an important option,
e.g., for cases where one wants to be able to dynamically change the
transport for testing or different kinds of deployment. For now, we
default to this transport in the library (via `DefaultActorTransport`),
but we may very well want to eliminate this because it will be
ambiguous with client libraries that vend their own
`DefaultActorTransport`.
The _Distributed module is being introduced in Swift 5.6, so its definitions need to come with matching availability. (We don't have version numbers for the associated OS releases, so we need to use placeholder 9999 availability for these.)
The concurrency runtime now deploys back to macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, which corresponds to the 5.1 release of the stdlib.
Adjust macro usages accordingly.
