Serialization of flexible types supported
Minor. Improved usability of CustomTypeVariable API
Minor. Getters replaced by properties
Minor. [platformStatic] used
getAllOverriddenDescriptors() moved to DescriptorUtils
Advancing ABI version

Related changes:
 - serialization of flexible types supported
 - Intrinsics.checkExpressionValueIsNotNull() added
!!! Tests for primitive operations
~~~ Disable failing backend tests