* add supervion for stages and built in ops run by interpreter
* add supervision for mapAsync and mapAsyncUnordered
* add supervision to groupBy and splitWhen
* reference doc for scala and java
* FlowMaterializer is now the actor independent interface
* ActorFlowMaterializer is the actor based interface
* MaterializerSettings renamed to ActorFlowMaterializerSettings
* impl.ActorBasedFlowMaterializer renamed to impl.ActorFlowMaterializerImpl
* Optimizations included in ActorFlowMaterializerSettings
* Note that http is using FlowMaterializer in api, but I suspect that it
will currently only run with a ActorFlowMaterializer
* StreamTcp and Http extensions now return Flows and Sources that can be materialized later
* Flow can now be completed with another flow to be turned into a runnable flow
- Switches from using size-of-1/2 Vectors to using Lists
- Fixes an issue where processorForNode wouldn't use the dispatcher form the settings
- Adds a dedicated Collect fusion op
- Adds various simplifications to ActorBasedFlowMaterializer
- Adds FIXMEs where appropriate
- Switches `grouped` to use a VectorBuilder
- Adds support for `scan`
- ActorBasedFlowMaterializer now uses Iterator instead of head+tail decomp on Seqs
- Identity and Completed Transformers are now cached
- Adds dedicated AstNodes for all combinators
- Adds a hook-in point for fusion in `ActorBasedFlowMaterializer`
- Adds support for `Operate` an AstNode with a function that create a fusing.Op
- Adds experimental and slow optimizer as a PoC
- Adds verification that Ast.Fused does not exist in optimizer input
* this is the first step, to enable safe FlowMaterializer that can be shared
* had to remove Log because of missing context (we need another solution for log)
* naming of the actors have changed, should be revisited
