3.0.0 Beta 4

Note: This is a pre-release, there might be breaking changes in the final stable version.

• Mark Future template parameter as covariant
• Add compatibility with revolt/event-loop v0.2.x
• Improve exception message of UnhandledFutureError
• Cancel DeferredCancellation when destroyed (#382)
• Rename combinators, introduce CompositeLengthException (#383)
  • race → awaitFirst
  • any → awaitAny
  • some → awaitAnyN
  • settle → awaitAll
  • all → await
  • The old names have been kept for a migration phase, but will be removed before the final v3 release.