Minor refactoring in CodegenAnnotatingVisitor

Create a private helper method
Fix StackValue.coerce void to Object

We can only put Unit.VALUE on stack if we're requested an instance of Object or
Unit. In other cases (arbitrary objects / arrays) we put null instead
Generate proper generic signature for closure classes
Increase ABI version

Due to changes related to FunctionN now being interfaces, FunctionImplN
classes, KFunctionN, callable references