0.26.0

• Stable, null-safe release.
• Add takeLast (thanks @ThomasKliszowski).
• Rework for retry/retryWhen:
  • Removed RetryError.
  • retry: emits all errors if retry fails.
  • retryWhen: emits original error, and error from factory if they are not identical.
  • streamFactory now accepts non-nullable StackTrace argument.
• Update ValueStream.requireValue and ValueStream.requireError: throws actual error or a StateError,
  instead of throwing "Null check operator used on a null value" error.