0.26.0: Structured concurrency

• Major rework of kotlinx.coroutines concurrency model (see #410 for a full explanation of the rationale behind this change):
  • All coroutine builders are now extensions on CoroutineScope and inherit its coroutineContext. Standalone builders are deprecated.
  • As a consequence, all nested coroutines launched via builders now automatically establish parent-child relationship and inherit CoroutineDispatcher.
  • All coroutine builders use Dispatchers.Default by default if CoroutineInterceptor is not present in their context.
  • CoroutineScope became the first-class citizen in kolinx.coroutines.
  • withContext block argument has CoroutineScope as a receiver.
  • GlobalScope is introduced to simplify migration to new API and to launch global-level coroutines.
  • currentScope and coroutineScope builders are introduced to extract and provide CoroutineScope.
  • Factory methods to create CoroutineScope from CoroutineContext are introduced.
  • CoroutineScope.isActive became an extension property.
  • New sections about structured concurrency in core guide: "Structured concurrency", "Scope builder" and "Structured concurrency with async".
  • New section in UI guide with Android example: "Structured concurrency, lifecycle and coroutine parent-child hierarchy".
  • Deprecated reactive API is removed.
• Dispatchers are renamed and grouped in the Dispatchers object (see #41 and #533):
  • Dispatcher names are consistent.
  • Old dispatchers including CommonPool are deprecated.
• Fixed bug with JS error in rare cases in invokeOnCompletion(onCancelling = true).
• Fixed loading of Android exception handler when Thread.contextClassLoader is mocked (see #530).
• Fixed bug when IO dispatcher silently hung (see #524 and #525).