Cancelation Tokens

API changes

• Added Proto.Promises.{CancelationSource, CancelationToken, CancelationRegistration} structs which can be used for synchronous cancelation callbacks.
• Added optional CancelationToken parameter to Promise.{Then, Catch, CatchCancelation, ContinueWith, Progress, Sequence, NewDeferred}.
• Changed Deferreds to structs with an implicit cast to DeferredBase (DeferredBase.ToDeferred(<T>)() to cast back explicitly).
• Removed DeferredBase.Cancel, replaced with the cancelation token.
• Removed Promise.{Cancel, ThenDuplicate}.
• Removed Proto.Logger, replaced it with Promise.Config.WarningHandler.

Behavior changes

• Changed Promise.First to suppress rejections from any promises passed in that did not result in the rejection of the returned promise.
• Change Deferred.Reject(null) to convert to a NullReferenceException on any T. This means Promise.Catch<T> will never give a null value. This more closely matches normal throw null; behavior.
• CancelationSource.Cancel(null) now converts cancelation to a non-value. This mean if Promise.ReasonContainer.ValueType is not null, Promise.ReasonContainer.Value is also not null, and if Promise.ReasonContainer.TryGetValue<T> succeeds, the value will not be null.
• Removed requirement to include Proto.Promises.Await for awaiting promises in an async function.
• Check to make sure RethrowException is used properly, change to InvalidOperationException if not.

Optimizations

• Deferreds are now structs, removing the memory overhead of classes.
• Added a static cached Promise.Canceled() void in RELEASE mode.
• When canceled, promises are now unlinked from their previous.
• No longer check if promise is canceled when doing internal operations.
• Removed internal skipFrame calculations since that's now handled in Internal.FormatStackTrace in DEBUG mode.
• Pass by ref to pass large structs around internally more efficiently.
• Less memory used in Promise objects with onRejected callbacks.
• PromiseMethodBuilder caches stateMachine.MoveNext directly instead of lambda capturing.

Misc

• Removed recursive type definitions so that Unity 2019.1.14 and older on .Net 4.X scripting runtime version will compile.
• Removed PROTO_PROMISE_CANCEL_DISABLE preprocessor checks since cancelations no longer slow down normal execution.
• No longer check for internal promise invoking when accessing Promise.{RejectException, CancelException}.
• Renamed asmdefs to include Proto in the names. Removed Utilities.asmdef.

Known Issues

• A Promise.Progress callback subscribed to a promise chain where the chain is broken by a cancelation token and continued with Promise.ContinueWith reports incorrect progress (introduced in v 0.9 with Promise.ContinueWith).