=str #16349 fix map + map optimization
Merge pull request #16349 from 2m/wip-16349-fix-map-map-optimization

=str #16349 fix map + map optimization
!str #15236 new transformer

* replace all Transformer with DeterministicOp
* use the IteratorInterpreter for strict if possible
* emit then become
* emit then finish
* termination emits
* FlowTransformerSpec
* rework types to work with Java API
rename and move things, add scaladoc
incorporate review feedback, and resolve remaining FIXMEs
revert the iterator counter in MultipartUnmarshallers
incorporate additional feedback