John McCall 186c53000d Introduce basic support for custom executors. ...

- Introduce an UnownedSerialExecutor type into the concurrency library.
- Create a SerialExecutor protocol which allows an executor type to
  change how it executes jobs.
- Add an unownedExecutor requirement to the Actor protocol.
- Change the ABI for ExecutorRef so that it stores a SerialExecutor
  witness table pointer in the implementation field.  This effectively
  makes ExecutorRef an `unowned(unsafe) SerialExecutor`, except that
  default actors are represented without a witness table pointer (just
  a bit-pattern).
- Synthesize the unownedExecutor method for default actors (i.e. actors
  that don't provide an unownedExecutor property).
- Make synthesized unownedExecutor properties `final`, and give them
  a semantics attribute specifying that they're for default actors.
- Split `Builtin.buildSerialExecutorRef` into a few more precise
  builtins.  We're not using the main-actor one yet, though.

Pitch thread:
  https://forums.swift.org/t/support-custom-executors-in-swift-concurrency/44425

2021-04-30 03:11:56 -04:00

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