Use Execution to run flows in REPL

**WIP, noting TODO's**

- [ ] Use Execution (from #915) to run REPL flows (rather than creating fake "Job"s)
- [ ] Add unit tests for functionality added in #918 using Executions.
I assume so. What is happening here is that the code in the repl needs to make it to the mappers/reducers. This is how that happens. we should make that more clear in the comments.

For instance, each function is a subclass of FunctionN in scala, but a different named class, so those classes need to get to your mappers/reducers.