Kotlin/Native multithreading

* Provides newSingleThreadedContext.
* Provides Dispatchers.Main on iOS, Dispatchers.Default everywhere.
* Coroutine references (Job), all kinds of channels and StateFlow are shareable across workers.
* Each individual coroutine is confined to a single worker.
* Update Dispatchers docs to account for native-mt changes.
* Multithreaded support in select expression.
* Fix ObjC autorelease object leaks with native-mt dispatchers (#2477)

Additional fixes:
* Fixed broadcast builder with different thread
* Fixed adding a child to a frozen parent job

Fixes #462
Fixes #470
Fixes #765
Fixes #1645
Fixes #1751
Fixes #1828
Fixes #1831
Fixes #1764
Fixes #2064
Fixes #2025
Fixes #2226
Fixes #2138
Fixes #2263
Fixes #2322
Fixes #2283

Author: elizarov

gradle.properties

@@ -3,7 +3,7 @@
 #
 
 # Kotlin
-version=1.4.3-native-mt-SNAPSHOT
+version=1.4.3-SNAPSHOT
 group=org.jetbrains.kotlinx
 kotlin_version=1.4.30